What year gmc?
On the spid, I think you would see reference to 157" in two spots:
1) the "w/base" entry at top of spid
2) the model number. Specifically, I think you would see "64" in the sequence which correlates to 157" wheelbase.
See the attached pic for an example (69 gmc
found here). Also, here is a
reference point for model numbers... While it doesn't call out 157" wheelbase I think you have enough info to put the pieces together.