The machinists may be getting the 235 head mixed up with the 230/250 integral manifold heads that are very prone to cracking. 235's really aren't any worse than most other engines and as long as you don't overheat it it probably won't crack. I have a '58 235 and a '62 261, both with lots of miles and both heads are in good condition.
You probably do have a sticky valve. You should be able to get a valve job along with a magnaflux of the head for less than $500, or just get a used head with good valves. There are plenty of them around.
