Further details:
"The Contemporary Diesel Spotter's Guide" by Marre and Withers (Withers Publishing 2000: I think there is a new edition since) gives the following lengths and truck center distances:
Pre-Dash-8 types for comparison:
U23/30/33/36B: 60'2"/36'2"
B23/30/36-7, including BQ23-7: 62'2"/36'8" (this is an oversimplification: I think these lengths are good for early production of these models but there were minor changes over their lives)
B30-7A (Missouri Pacific): 62'2"/36'8"
B30-7A1 (Southern): 62'4"/37'10"
B30-7A(B) (Burlington Northern): supposedly 62'2"/36'8"; I don't know whether this applies to both orders or not.
12 Cylinder Dash-8:
B32-8: 63'7"/36'7" (This is good both for the BN-painted 1984 demonstrators and for the 1989 production version)
B32-8WH (Amtrak): 66'4"/39'7.875"
16 Cylinder Dash-8:
B36-8 (GE test unit),B39-8 (Santa Fe-painted demonstrators), B39-8E: 66'4"/39'4"
B40-8, B40-8W: 66'4"/40'1.5"
Most of the length difference between the B32-8 and the B39-8E seems to be in the long hood, so look for hood openings for more differences: the obvious one is the traditional distingushing mark of 12-cylinder vs 16 cylinder GEs: six tall engine compartment doors on each side for the "small" engine, eight for the large. Production B32-8 and B39-8E seem to have the same number of handrail stanchions (at least on the left side), but spaced differently.