4.3L for the jimmy won't really work without alot of changes. Including the 5 speed tranny. This is because it was a 2.8L. The 3.4 Bolts right in, same wiring harness, computer, etc, etc.  The 3.4L is the fast,  best replacement for the 2.8. Thanks for the idea though. I get told that lots.  Yes, I also know about the 350 swap. But with the 3.4L you get a 20% power increase while keeping everything else the same and still get good mpg.
P.S. It will be a while before I get to the radiator. About another month. Don't worry about the crack. It has been like that for like 2 years now. I just keep antifreeze and water in the back. Check it almost every day and top it off.
later,
Josh