I know the "-2" models have a "porch" at either end to fill up the extra frame length. Did the SD45-2 have any longer hoods, thus filling up the "porch" at the ends of the frame?
The SD45-2 has a longer radiator compartment than the SD45, which has a longer radiator than the SD40. The 45 series also has a longer engine compartment than the SD40 and SD40-2--it still has 8 engine doors on each side, but the doors are wider. As a result, there are no "porches"--the platforms are the same length as a Geep of the same era.
So while the -2 frame was designed with the HTC truck in mind it was also designed to fit the larger prime mover and radiators of the 45
I think it was the other way around. If the 20-cylinder engine and larger radiators could fit nicely on the 65' 9 1/2" frame of the SD45, why lengthen it just to accomodate these features? I think EMD took advantage of the longer frame on the Dash-2 units to lengthen the radiators on the 45 series, thereby eliminating the need to have canted intakes. There was no need
to have the radiators that long--the radiators on the SD50 through SD70 are taller but much shorter in length.
I've got drawings of all of the above posted at http://trainiax.net/mescaleloco.htm