i have done searches on this and everyone said to check the compression and look for bubbles in the radiator. i did both and the comp. was 165-180 for all 8. i looked for bubbles and didn't see any, it was moving but not bubbling. when i drive for a few miles and then open the hood there is coolant that obviously came out of the overflow. if i pull the relief valve on the cap it seems to have way to much pressure and blows out the overflow. the temp is ok but it will keep losing coolant until there is not enough to keep it cool. 305 with 4bbl any ideas??

Leaking head gasket or crack in block or head causing air to be pushed into cooling system, maybe still possible without seeing bubbles.

Are you getting any coolant in the oil?

Have you replaced your rad. cap recently? The cooling system has to hold up to 15-16 psi in order to keep the water from flashing over to steam. If the cap doesnt hold pressure, youll get a boil over. Its worth a shot to try a new cap.

new cap and no coolant in the oil. if the heads or block were cracked or blown head gaskets wouldn't it show up on the comp. test?

You many not necessarily see a gasket leak or casting crack with either a compression test or a cylinder leakage test. A leakage test will have a better chance of finding a leak, but if the failure closes when the castings cool, and it only leaks when fully heated, you could have real trouble diagnosing it. I had a head gasket fail on my truck with similar symptoms. Slow coolant loss, excessive system pressure and relief, and no apparent compression loss. The problem became obvious only once the head was pulled.