Just a headgasket or signs of something more?

[Taylorb26]

I bought a 94 k1500. 350 but the block is apparently from a 79 pickup bored 40 over and has a little better cam. I wish I some sort of specs but I don't...
I got the pickup running decent after fixing 2 vacuum leaks, cleaning the throttle body and replacing the spark plugs, which were all fouled.
Well after running it for about 5 minutes I opened the radiator and coolant shot out. Shouldn't have gotten hot enough to build that kind of pressure so obviously it's a headgasket issue sending combustion pressure into the cooling system. I've seen this a bunch before so I did my plastic bag trick but found something unusual for me. Usually when I wrapped the bag around the rad neck it just blows up with air. This is sucking air in and blowing out. It's going along with the intake and exhaust strokes. Is this usual or could this be a sign of worse things than a headgasket? Were tbi heads known to crack? I've gotten some pretty clueless answers as to what was actually done to the vehicle so I'm wondering if I should just swap in a healthy 383 stroker sbc with a carb and set up a tps to keep the nice rebuilt 4l60.
What did you guys do with an unknown engine

 

[Taylorb26]

Oh also the guy said it was professionally times by a gm tech.. well there is no timing tab on the thing and it kicks back on the starter. Sounds way too advanced.