The axle from the '01-'07 Silverado will not work, as those trucks are IFS. The F250 axle might be a bit wider, but should be prety close. You would want to use one from leaf spring 250, the Twin Traction Beam axle (coil spring) won't work and is not as strong.
Also, just my $.02, if you're planning to do any towing, you might consider a Deisel. Far more efficient for towing. You'll want an engine that makes good low end torque, not high hosepower, as it's torque that gets you going. Deisels have that in abundance. Another option would be a 383 stroker if you want to go gas engine. But stay mild on the cam, no need for a wild cam in a towing engine.
