You could do a hard fill on the block.  You could use a ceramic sealer on the water jacket.  Or you could stitch seam the crack.  I'd probably do the ceramic seal as my first choice, with stitch the block as my second.  The hard fill has some cooling considerations where the other two don't.

That's really not that bad.  If it's not cracked in the crank saddles, you are usually able to repair the block.  Lastly, don't trust your eyes.  Get the block Magnafluxed.  There could be other cracks hiding that you can't see.

A laser thermometer can tell you a lot.  You can get them for less than 20 bucks.  That will help you understand whether the problem is on the cooling side, or the engine side.  With that, you can look at the temperature deltas between the radiator inlet and the radiator outlet.  A normal spread is around 10-15 degrees.  A narrower delta, like less than 10 degrees, means you don't have sufficient cooling capacity which can be too small or inefficient of a radiator or inadequate airflow through the radiator from a faulty fan or drive assembly.  A wider delta -like too cool- can point to restricted flow, meaning coolant is spending too much time in the radiator. You also want to check the whole surface of the radiator.  You are looking for wide temperature variations that tell you where there are restrictions. 

For the engine side temps, you want to look at what temperature you get at the water pump inlet and the manifold temperature ahead of the thermostat.  The delta is very similar.  Retarded timing or a lean mixture will give you a wider delta.  Low block pressure can do that too as low block pressure can allow steam pockets to form and once you have steam pockets, you aren't cooling that piece of metal.  You get low block pressure from a slow or inefficient water pump.  Also, don't run straight water of any kind.  Water has high surface tension and the electrons don't like to get friendly with metal components so you get poor surface contact.  Use antifreeze or a surfactant to break the surface tension.

What is the projected life of a solid roller on the street?  ...

This comes up a lot.  The answer depends upon where you ask the question.  I look to a bigger brain trust with Speedtalk.  Here's a link to this question:  https://www.speed-talk.com/forum/viewtopic.php?t=43176

If you search the site, you'll see it's not the first time the questions has been asked and bench raced.  In that particular thread, there is a response from CamKing.  He is a cam grinder and designs custom pieces for many of the NASCAR and other race series teams.  But with anything, it always depends on how the particulars stack up for you.  Maintenance is very important and has a lot to do with valvetrain life when working with a solid roller.

That's the sort of thing that would be a perfect brazing candidate.

And then, there is the air cooled VW or the Porsche flat six.  Bedplates are also common on diesel engines like the earlier Ford Power Stroke.

Just something else to consider:  I replaced my 5/16 steel line with 3/8 cunifer.  While cunifer is vastly easier than steel to bend, it's still not as easy as running a braided line but it will last forever.

The zinc in the carburetor is probably disintegrating.  Zinc powder is a dark grey/blue color.  While gasoline is generally neutral, there may be additives that have become or are acidic, particularly high levels of sulfur or bad storage conditions.  It's hard to measure that so you end up accepting the symptoms as the evidence...unless you can send the fuel off for analysis, which may be worthwhile.  Blackstone does fuel testing. 

...

I do have to ask though, what do you think you're gaining by switching to a diaphragm pressure plate?  There is work involved to make them work with an older factory vehicle, so I'm just curious why you think there's a benefit there.

It is a preference.  I know they work and they have always worked good for me even in racing conditions.  The work involved is nothing more than removing the assist spring and setting up the pedal travel to keep the spring from going over center.  Both are quite easy.  You are misjudging benign sarcasm for snark.  Not everything is an assault.