[Jack Gifford]

Welcome to these forums, and cheers to the GMC owner! I've got a '59, but it's had a later Pontiac swapped into it.
Tell your friend that basically all Pontiac exhaust manifolds interchange (other than high-performance round-port stuff). There are many passenger car manifolds that would work for him, he just needs to look around at some Pontiac engines. I believe the '72 350 in my truck has its original manifolds, and they work real nice for duals, and clear the steering box and everything good. However, my '72 engine has the block-mounted starter, so I can't guarantee the driver's side would work with his bellhousing-mount starter (although it appears it would be okay).