I'm with the others on the manifold alignment. Don't try to separate the intake and exhaust sections. I don't remember if GMC used the gland rings/sleeves in the intake ports, but if so, they are difficult to align. If the gland rings are necessary, you can see a counter-bore in the intake port of the head, and also a matching counter-bore in the manifold. If you don't have the counter-bores, don't try to use the rings.
If you had the rings when you disassembled the head, and you are using them, I think your trouble is purely alignment. Loosen the manifold quite a bit and move things around. If the exhaust pipe is still connected to the manifold, it is all the harder to get everything aligned. Your problem was common on Chevrolet engines with the gland rings. It just takes proper alignment to get everything together. And you get better at it with practice.
I might also add that there are/were quite a few Chevrolet engines running around without those rings. And some GMCs too.
Harvester
