GMC used the 287ci Pontiac V8 in the 1955 pickup so parts should be available for a straight out swap. You don't say if it is the granny 4sp you want to use but if just a 4sp then many many GTO's, Firebirds, Trans Ams, etc came with 4 speeds on 389 engines.
Being ridiculously "picky", no Firebirds, Trans Ams, etc came with a 4 speed on a 389 engine; as neither were introduced prior to the demise of the 389
Of course, the 4 speed with the 400 will fit the 389.
Jon.
