IMO that would be one colossal step backwards. While IH V8's have great low speed torque and a truly superior cooling system design, that's about all they have. Mileage is terrible, HP is terrible, they expensive to do major internal repairs on, they are the physical size of a big block Chevy, and they're at least as heavy as the 6 sitting in the engine bay now and maybe heavier.
I know that it's very "cookie-cutter", but a 350 Chevrolet is better in every aspect except possibly the low speed torque and definitely the cooling system - and that can be changed with a focused engine build for a lot, lot less than an IH engine can be built to have more HP (you've never solve the better efficiency riddle - the cyl heads are stacked against you and no amount of porting will fix the basic design flaw).
That said, the IH engines use a bellhousing that is significantly longer than pretty much any other engine. Their manual trans' input shafts are significantly longer than any other. Visit the AA tech pages if you want to know those details, but that means that the IH b/h isn't adaptable to the existing H-4X trans. And the IH b/h bolt pattern is unique to their engines so you aren't going to bolt a GM bellhousing on and then adapt it to the OE trans. So likely your best option is to adapt the T-18 or T-98 that is bolted to the IH engine to the Split Case t/c. In doing that you will go from a surprisingly nice shifting truck trans to a very typical shifting truck trans that lacks a synchro in first gear. Although if you're lucky your donor may have the 6.32:1 first geared T-18 instead of the more common 4:1 geared T-18.
About the only thing positive that I can say about such a swap is that once in there, it should be extremely simple to swap the IH gasser for the IH built Ford 6.9L or 7.3L IDI diesel.
