Actually, I wouldn't put anything on the cap - it is designed to blow off at a certain pressure, and sealing it may allow too much pressure to build up.

You may just need to replace the cap, as the seals on it eventually crack, or the spring may have failed (works like the thermostat does too. If the neck visually looks fine, it is probably just the cap - ~$10 from the typical parts place, $21 from the dealer (may be special order) for the right one. I just replaced mine with the one from the dealer yesterday because a shop told me the seal was bad. That may not solve your overheating problem though, but it is something to check.

If the neck is damaged, you might find a place that will repair it, but I would probably replace it if the damage is significant. You can very easily test the thermostat by taking it out and putting it in a pot of hot water.

When you did the rebuild, did you redo the pistons also? Did you do a compression check on the cylinders? What you describe can be worn rings and it could give symptoms of a blown or leaking head gasket - when you do the head, I believe it is normal to replace the rings at the same time as well, because you're in there already.