Now you don't have a replacement head, you have a replacement head and a new manifold, which (I'm guessing) does not exist yet. The current GMC manifolds have the new port size, but unless you want to re-align at least 2 ports it won't fit the head. The 1st to last cylinder center distances are 21-5/16" (235) and 22-1/4" (302).
I agree that the GMC head is an improvement, but not compared to anything 1955-*. The original 12 port heads are retro vs. the better OEM engines of that time.
The "tweak" I recommend is what has already been done many times: make a wedge out of it, and use SBC parts.
There's also a latin phrase that I believe applies here: materiem superabat opus.
