The frame parts from Trumpeter and Italeri:
These parts were the first parts, I modified, and I made some errors right away
The transfer case is placed in the same distance from the front of the frame on the M983 and the M985 respectively, indicated by the black arrows in the picture. This also means all mount points for the suspension units are placed in the same way on the forward part of the M983 and M985 frames. So I should have made the cut in the area indicated by the dark green snake pattern line. So the orange areas indicate my cutting errors, which should be ignored. I will do better the next time
The important point here, is that you need to add lenght to the short M983 Trumpeter frame two times, since the M985 frame is longer than the M983, but the difference in lenght is not evenly distributed.
There has to be room for the crane, thus there is a need for some lenght after the rear suspension unit, and there is a need for some extra lengt between the two suspension units, since the M985 has an extra box between the two suspention units, which is not present on the M983.
The suspention units are mounted on the frame by the attachment points indicated by the light green and red lines on the Trumpeter parts. These attachment points match the light blue and yellow lines on the Italeri parts.