The track width on a modern half-ton is much wider, and as mentioned already you should measure the widths to see the difference. The wheelbase of the 1990 should be 131.5", while you need 116-117". so you would be cutting a lot out of the frame. This would involve a lot of measurements (and probably a jig to line everything up), as well as welding the cut line and then plating over this seam and welding that as well.

No offense, but if you have to ask how hard it is it might be a bit over your head. It's a lot of fabrication and is critical that it be done by a good fabricator and welder.

In addition, the frame swap requires a lot of fab, and these 3100 cabs are much easier to swap onto a S-10 frame, if you're set on a frame swap.

Last edited by 46gmcpu; 11/05/2012 11:21 AM.
1946 GMC Pickup - S-10 Frame, 455 Buick, TH400, original patina.

My 46 GMC on Photobucket