I'm thinking that the main difference between a 3100 frame 3600 frame and 3800 frame from the front cab mounts forward is the thickness of the frame rails.
The rail might be a little taller from top to bottom right at the point above tge axle center line but some dilligent calculating and slight mods. can get around that.
This is on my my photo bucket page but even with the assembly manual in front of you the numbers are hard to read.
This is 3100 and one of us would have to take some measurements on our 1/2 ton frame right at the spot where the axle runs under it and the independent would mount to show the measurments that you could compare w
ith your frame.
Center of the front wheel to front of the front bumper is 33-7/8 on 3100 and 3800
https://www.gm.com/content/dam/compa...olet-Truck.pdf
This screen shot shows the frame rail thickness difference. Also the main part of the rail is 7 inches rather than 5-3/4, More beef, all on the bottom side.