Problems? Heat and Pressure.

So I have not owned this car long. 69 Plymouth Valiant 440, 727. Champion radiator and cap. When I got it it had no exhaust so I put flowmasters with full 2 1/2 pipes all the way back. Then that is when the problems start to creep in. Runs hot!!! 230 and it might go more but I shut it down. So I decide to change the easiest thing the thermostat. I open it up and it has a 180 in it already. I drill the 1/8 hole in the new one and button it back up. Start the car and let it run until the thermostat opens. Keep topping off the coolant as it is burping. At this point it never goes over 185. So its been idling for 20 mins alteast. I start to notice I am smoging out the neighborhood. I decided to put the new radiator cap on and see if it will get hotter than 185. 3 mins later a bottom seam on the radiator is gushing out like the falls!! I feel that its building no pressure so I remove the cap and the leak stops!! Hello! So I shut it down and let it cool and it seems to build pressure as it cools. This morning i remove the cap and I get the hissing pressure noise. I decided to borrow the radiator pressure tester from work and cold at 30 psi applied i can not get this radiator to leak again? I test the old cap and it releases at 14 psi. I think I have a head gasket problem.