Started the car and it was running for about 20 seconds, went to go open the radiator cap and it built pressure somehow, so I bought this kit that detects combustion gases in coolant and it failed the test.

The car doesn't have milkshake in the oil, no oil in the coolant and no smoke out the tail pipes, just combustion in the coolant. It's gotten worse over time, if you rev the engine with the cap off it'll spill over coolant and create a ton of bubbles. If you drive it for long enough it'll create excessive pressure and try to blow a hose off even though it is not boiling, 220 is the max its ever gotten before I shut it down, it wont cool itself down. What's the likelyhood of it being a cracked head as opposed to just a head gasket? (217/083 TPI 350 heads)

Head gaskets are already in the mail, I bought the severe duty permatorque gaskets FEL-PRO 501 SD. Kinda paranoid its a cracked head, I think I wanna take them to get tested

 

Not all that likely butt certainly not impossible or even totally unlikely.

More likely, esp if the gaskets are steel shim or very old, is one of those.

Eeeeeeezzzzzzzzy enough to have heads tested. Do that if there's not an obvious gasket failure.

 

I would agree that the gasket is the most common failure point, but you won't know for sure until you pull the heads and inspect.
If you haven't pulled the heads yet, you could try a leak down test on each cylinder, it may give you a better idea of where the issue is. Could be useful in guiding your inspection if you don't see a clear issue when you pull the head.