Borbas,
If the Pontiac engine is inn good shape it's a simple install since the GMC's of that same era had Pontiac's in them from the factory. Here is a photo of a 400 Pontiac in a 55.2 Chevy 3100.

[img]https://i.postimg.cc/bZXZmT7Z/55-pu-eng-bay-LS-lift-Int-fuel-tank-004.jpg[/img]

[img]https://i.postimg.cc/gxQbVSQ7/55-pu-eng-bay-LS-lift-Int-fuel-tank-005.jpg[/img]

Evan