Will it fit? I'd say yes, quite well. The IH rear frame width is 33 1/2". The Chevy rear frame width is 34". With 1/4" steel plate to fill the gaps it will fit very nicely. By welding the 1/4" plate to the frame, I can strengthen the frame as well.
The track width from outside of tire to outside of tire (using Chevy 15"X8" wheels with 255-60-R-15" tires) is 74". The rear fenders measure about 77" inside to inside. This should work very nicely.
Now I can get rid of the remains of the donor truck. :)