Buck,
What tranny was originally in your '48 GMC?
Was it by chance a floor-shift 3-speed?
Column shift 3-speed?
The earlier tranny pattern on the truck (and car through'54) had the top two holes fairly close together(?).
The aluminum bell housing in the bottom photo appears to be cast in such a way that it can be drilled to accept any of the post-54 trannys.
If the bolt pattern on your bell is different from the one on the green bell housing in the picture, than I'd say you have the early pattern housing on your engine, which is very limiting: you'd need to change to the '49 to '53 bell housing to mate the later trannys.
