I think you would be adding alot of extra work by cutting and welding the frame. In your original post you said "However, after taking a few measurements I have found the 75 frame width is exactly the same as the 57 at the point at which I cut it."
That being the case, why not just unbolt it from the donor vehicle and bolt it to yours? If you want extra strength just weld it after bolting it on and forget about it. You gotta remember that there are thousands of C10's around from the factory that had the IFS just bolted to the frame with no problems.
What I am thinking of doing is switching the front ends from the firewall foreward. I cut the C10 frame at the firewall and it is the same width as the 57 at the firewall. However, where the A-arms and other IFS parts mount to the frame, the C10 frame tapers to be narrower(about 28.5" I think) whereas the 57 frame is 34" bumper to bumper.
I think these
pictures will help visualize what I am dealing with.
