I'm not sure what you are trying to change/improve? What year(s) are the parts from or trying to end up being?
Assuming that the frame is a 3600 (narrow at the rear compared to the 3100 frame) I don't think the cab or front end is different between the two frames but the bed to frame mounting points are different among other things.
I assume that the rear axle change was done to achieve a different axle ratio but what is the drive train in front of the axle? The 3600 had a two piece drive shaft with the front being enclosed and the rear portion being open. Width of the axle and resulting tire relationship to the bed/fenders is important but not apparent from the picture provided so far. It appears that there may not be shocks on the rear axle, if that is correct it is something you will want to address.
