Found a block only 120 miles away from home.
Standard bore size 4.05".

My machine guy borrowed me his sonic tester, so I took it with me to check the block out. Measured all the cylinders, 12 spots each.
One spot, pointing toward the front of the block, is 0.08", all the other measurements turned out to be 0.14" to 0.24".
Am pleased with that.

Unfortunately, although the guy I bought it from, is the owner of an engine machine shop, too, he did not have a straight edge. So I could not check the alignment of the main bearings.

But, very pleasantly, although the block looks like oil changes weren't on the regular menu, all the water jackets look almost new. Mister no-oilchange must have at least used proper coolant.

Sorry if this has been covered and I missed it, but if he used a sleeve to calibrate his sonic checker, that will not be accurate to check the bores.  A sleeve is a much different material than a cast iron block.  My sonic checker requires that you select the proper material for an accurate measurement; mine says to use the grey iron setting for cast iron blocks.  Check calibration on the rear block rail, as Ross suggests.  If the transducer is curved it won't matter, as long as the transducer is held at a 90 degree angle to the flat surface.

Thank you Jay. That is a good hint. I will tell him to do that.

BTW before I had anything done to the block I did a crack check that turned out ok.

Thanks guys, appreciate the input.

Machinist is an engine builder (I only call him "machinist" here and there because I build my engine myself, I pay him only to bore/hone, square the block and plane the heads, cut the valve seats), specialized only in US American V8 engines and he does it for decades. He and I get along well and I kinda trust his expertise.
However, I sure wish you guys are right, which makes me wanna double check the measurement for sure.
Currently, the block along with rotating assembly and heads are still at his place, 3h drive away.

I will pay him a visit as soon as I can and measure the block together with him. I will then triple check on the calibration of his device, putting your tips to use.

If the cylinders were indeed that thin, I couldn`t imagine that engine wouldn`t have split all the cylinder walls in normal use. Or how they could all possibly so thin at 4.05" standard bore, unless maybe it was a boat engine that was using salt water cooling, and that ate away more of the cylinder walls from the backside. Many years ago, when I worked at an engine shop, we had a Chevy 11 style 4 cylinder engine (Think 194 6 cylinder missing 2 cylinders), that was a boat engine, that was getting coolant in the oil. When the water pump was removed, you see pinholes that went completely thru the cylinder walls.

My machinist and I are both evenly amazed about how this engine did not crack.
He calibrated his ultrasonic device twice (on an old sleeve he has laying around). It has a rounded probe.
I took it out of an unmolested, original, bone stock 1970 F100 that we bought in Oregon and had shipped to Germany. Casting numbers however, indicate it is a 1972 block (and heads). Untracable if this engine was maybe used in a boat. But I consider it unlikely. Engine was bone stock. Even the rod and main bearings had the FORD stamping in them.
The truck it came out of is in pretty good condition, bed has almost full original paint in it, but a hitch and trailer brake was installed, so maybe a lot of towing? Noone knows.

Take a vacation to the US and find a 390 block you can use. Build a wood box around it and ship back to Germany. (May be stuck in customs for awhile)
Probably wouldn’t cost much more than the asking price for that other block and you get a vacation as a bonus!

Not an unreasonable idea.
Against it speak:
I don't have a suitable amount of free days from work until maybe September.
I don't want to (can't) spent that kind of money. Neither for a used block in Germany nor for a vacation with block-purchase ;-)

Hello from Germany,

my machine shop measured the cylinder wall thicknesses of my 390 FE block before starting to work on it.
The thinnest spot he measured is 0,4mm (0.016"). Cylinder 4, thrust side, adjacent to the freeze plug. Other spots on cylinders 1, 2 and 3 are around 1,2mm (0.047").
Cylinders 5, 6, 7 & 8 all have thin spots at around 1,8mm (0.071").
He said this is due to core shift and heavy rust in the coolant passage.

The block is standard size 4.050.
Target bore is 4.080, already bought new pistons and rings for 4.080

Goal for the motor is:
Torquey truck engine, max rpm around 5500

• 9,5 - 10,0 compression
• New Forged Sealed Power pistons
• New Molnar rods
• New Edelbrock aluminum heads
• New Roller cam from @blykins
• New Roller rockers from @blykins
  
• New Performer RPM intake
  
• New Headers
  
• Crank is 0.01 under and has been straightened and meticulously balanced with and without damper/flex plate
  

I have already bought ALL the parts, everything, spent well over 10.000$ in parts already (remember I have to have everything shipped to Germany and pay German VAT and import taxes) and budget is getting thin.

• The block has all the oil mods (by myself)
• The block has already received a line hone for the mains (@ a different machine shop)

My goal is perfection. I am 52 years old and intend to never open this engine ever again once it is finished. Do it once, do it right.
Boring/honing will be done with a boring plate.

Considering all this:
Does sleeving the block make sense?

My current machine shop (does only American V8s, nothing else) has never done sleeving and is sceptical.
He won't do it because he has never done it before.

Another machine shop, also specialized in American, V8s, specifically Ford, told me on the phone that doing 8 sleeves is absolutely no problem, has done it many times before, never an issue. Even on race engines for historic motorsport (Cobras, GT40s, Mustangs) well over 500HP, no issue.
He would put 8 sleeves in my block for reasonable money.
He said something about sleeve wall thickness of 5mm (about 0.197")
I asked him if it wouldn't be a problem that the sleeves will partially be in coolant and partially not: He said "No problem"
I asked him if the block will distort again during sleeving process, hence destroy the line honing of my main bearings: "No problem"

I am sceptical. What is your opinion?

Please keep in mind that finding a spare block in Germany/Europe is not so easy. Currently there is only 1 FE block in standard bore being offered, rough condition, the guy wants 1.600€ (about 1.900$ !!) and I don't know if this block is cracked, and how thick thy cylinder walls are.

Any thoughts are appreciated. Thanks guys.

best regards
Matthias