U raise a good point was it done at the factory?
Years of cyclical stresses and then one day it's there. It might have had a small crack there with the other spot but it wasn't noticed. I don't think they had a good method before magnaflux.
ps: cracks can be hard to see why I recommend the dye penetrant. eg; A spare motor I bought as rebuilt was resleeved in 3 cylinders by another engine shop, but my engine builder found a fourth bad one. It didn't show up until he cleaned it. There's plenty of good engine shops in the Charlotte area with the NASCAR business. I'm 8 miles from city limits.

I corresponded with the spectra chrome guys, got the video, etc.last year. I'm interested to use their mirror aluminum finish on part of my hot rod but I'm far from paint.
You spray the base then spray water on it and it changes to "chrome". Once it cures, you coat it with clear. I'm told you can use other people's clear but they won't guarantee.
There was an episode with a '53-4 Packard(patrician?) on one of the Velocity channel custom auto shows, not sure which one, but I think it was the Detroit guys and it looked good at a distance. But it's not cheap either. It came out to >>$20/ft2. And you have to buy a minimum qty. They'll want to sell you an application machine also. So I'd go to a shop that does it first. Just depends on what you want. Beats aluminum spray paint some use.

You need to decide if you want show quality chrome or not and how fast you want your parts back?
Paul's is very expensive. Advanced Plating (nashville) beat them 50% on price. The delivery is not fast, expect >8 weeks, maybe 14. They did all mine ~$5K and it's flawless.
The bumpers I'll do elsewhere when I get to that point. It's too much money to show chrome them on a common Packard. If you have a V12 or something >$100K in value go for it.
There a place in TX that claims 2 week turn around but no experience. No info on cost. They're in Hemmings. You have to dip your toe in and test chrome vendors on a nonessential part and see if you like it.

Good luck.
Don't want to do all that, spend $$$, and put back and find problems inside later. Now's the time. Make sure they dye penetrant test the whole block. Where's there's one...
Or you can do what they used to do, put a some "additives" in and sell it fast!
They should've honed the cylinders after boring so to have vertical lines would not be expected. If what looks like a crack in cylinder is not and a "groove" from machining somethings not right.
No 502 but maybe able to fix you up with a lo miles 288 L8! Great boat anchor.

"It doesn't seem to be firing"(#4)
Do you mean not making as much power instead?
It's not clear why you suspect this cylinder?
Firing can be easily checked. So if it's not firing, as you say, it's an ignition issue that needs fixed and is not puzzling. {I like to take the conductance type timing light and put on the affected wire and check the pattern of light flashes. Or you need an analyzer.}
If ignition is good, checks to spec, then it's something else.
There's alot of reasons beyond that, so you need a methodical approach.

Is that another crack in the top of the cylinder wall at "9:30"?
You probably need to dye penetrant test or magnaflux the whole enchalada. (takes care about the ones you're not seeing.)
It's a crap shoot welding cast in an engine block in critical areas. Any "good" engine builder could tell if it's repairable or not. From your request, I'm assuming your guy has stopped work and given up, not a good sign.
I'd be looking in parallel for another low miles motor/block, just in case. You get into the cost of repair vs replace. And there's no guarantee cast repairs will hold. No telling if another vintage block will be any good either, if it's a common problem one.

CAAAR uses a ten point system and has a detailed description for each.
http://www.caaarguide.com/id1.html

FYI - technically, vacuum is pressure.
We refer to gauge pressure (psig) as above atmospheric. Where as absolute pressure (psia) is measured from full vacuum, P=0 psia.
"Vacuum" is fractional atmospheric pressure = 14.7 psia, 29.92 inches Hg, 0 psig, 33' H2O.
Eg, 5 psia = 20 inches of "vacuum" = 10 inches of Hg = -9.7 psig.
It just depends where measured from, i.e. relative measurement.
We use common terms, "vacuum" & "pressure" to conveniently distinguish above/below atmospheric but it's all pressure.

Only that the older restoration may have been done when parts were still available, original or NOS, where a new one may have some repro parts or mods. ie more period correct, paint, etc.
It also means that it's not in original untouched condition which some desire and value over any restored.
But nothing is exact in this trade. You have to go look and compare to the comparables. And know what the market is to determine value.

If you operate outside the normal design parameters, expect issues. This is a vintage auto, not a modern rally car. You may need to stop occasionally to let things cool rather than drive on, out of range. You may need to modify/adapt your selection of lubricants, coolants, etc.
I wouldn't necessarily be concerned seeing 100 C coolant temperature, if the outside temperature is as you describe. They're pressure cooling systems, albeit low,~7 psig, so can operate slightly higher than atmospheric BP(100C)
Energy travels from hot to cold, so it has to overcome the increased ambient conditions by increasing temperature to expel heat. This is normal.
Gauge working/accuracy is a separate issue.

A fool and his money still applies.
It's easy. Chip Foose does it in a week on TV! You just take this old part out and put this new part in. DAH.

What I don't understand, maybe have contributed to by trying to help occasioanlly, is they all ask questions on these forums rather than seek the correct help. It's appropriate for some things but not all.