3.8 engine questions

[MY PRIZE]

Hey guys, I know this isn't a mustang question but I wanted to share this in the hope that someone out there is a better mechanic than I am so I can get this car put back together.
Heres the problem, my son has a 90 T bird SC (3,8 super charged engine) the motor blew a head gasket on nr 3 cyclinder so I changed it out and put the engine back together. After running for about 30 minutes the engine began to over heat. I turned it off removed the t stat, checked the radiator flow, ran the engine again and after about five minutes it overheats again. Seems that it works ok if the rad cap is off but as soon as you put the cap on the upper rad hose it gets so hard it looks as if it might blow, plus engine temp spikes. I dissassembled the engine to make sure that the head gaskets were on correctly (they were). Since then I've checked all the engine coolant passages for blockage (found none) checked the water pump (ok) had the heads checked for cracks (none). I'm ready to put it back together but hate to do this being as we have'nt found the problem.
Does anyone has any ideas?
This is possibly a crack in the block somewhere? Someone told me that they thought it was an air pocket in the cooling system and that I didn't run the car long enough to get rid of it. What do you guys think?


Now: heres the next question, as anyone seems any ads for chrome valve covers and other engine dress up stuff for a 4.6? I can't seem to find anything in the mags. I would like to do some dress up before the spring car show season starts.


Thanks guys in advance for any help, I really need to get that T bird going so I can stop the ongoing taxi service.........