I had one just like it, twin 660’s were about two tenths quicker and 3 or 4 mph better than a single 1050.

Why would you bother?  If you want it original leave it alone or give it a stock valve job. If you want more hp and driving fun there are other places you can get a better bang for your buck.
By the way a bone stock 390 kind of sucks, when it gets high mileage and wears out you almost have more cost in keeping it stock than using aftermarket parts. There are plenty of aftermarket (stock looking) parts that can wake it up and put a smile on your face.

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!

If you’re seriously considering a 427 build for the car I would only add parts to the current 390 that could be swapped over to the 427. Things like headers, distributor, carb, valve train etc. A lot of parts fit both. I wouldn’t want to purchase new parts twice.

like NHRA Stock Eliminator, where hydraulic lifters are mandated by rules,

Rory, you do know that rule is superseded and solid lifters can now be run in any combination. 😁

Is the cam duration listed correct?
266/280 @ .050 is a healthy duration to stop at 6,000rpm, especially with tunnel port heads.

Mike, any test runs on the Mustang yet?  I see you added the leaf spring rear slider from Calvert. Do you know the theory behind how they work? Is it a better set up than stock style hangers?
Also I couldn’t tell if you have stock or reinforced spring perches. I was using the stock 9” perches and a couple of weeks ago at a double divisional race my 60ft went away and traction became erratic. When I got the car home the perches were smashed on the back side and gapped off the axel in front, my Cal-Trac U bolts also look bent. I’m pulling everything apart for repairs and was looking at the rear sliders.
Thanks, Mike Gray