Radiator Overflow Problem...

[PowerMan]

We have been having a problem with the radiator forcing coolant out. When we drive the truck (1962 W100 with newly rebuilt Poly 318) on the freeway it runs great and the temp gauge is straight up and down for about 15-20 mins and then it slowly moves towards the 2/3's mark and coolant blows out. We put on an overflow tank a while back to see if that would help solve the problem and now it fills the overflow up and blows it out of that. When we start the truck up, it seems to heat up to a little past the 1/3 mark very quickly. We asked the local radiator guy and he said we needed a 12-13 lb. cap (we had a 7lb cap on there before) and it still blew coolant out. We thought that there may be a problem with the thermostat so we put one in we knew that worked. That didn't solve the problem, so we took the thermostat out completely and ran it around like that and it still blew coolant into the overflow. We were then told that we may have a head gasket leak which would allow exhaust into the coolant jackets and cause the heating problem, so we replaced the head gaskets. That didn't help either. The next suggestion that was made was that there was problem with the water pump. After changing the water pump, it still forced coolant out.
We are at a loss for ideas.
The only thing we are thinking it could be is a cracked head.
Any suggestions are welcomed.
We are really pushing to get this fixed for next weekend's California Power Wagon Rally at the Hollister Hills Off-Road Park.

Rob