As said before, swapping a drivetrain can be expensive, but if you can locate an entire donor vehicle to strip, then most of the items would be from there and would help keep costs down.
I had the 4-banger in my CJ for 14 years. I went the route of putting on the Weber carb, better exhaust, better air filter ect., but I think that putting in the 4.3 was probably one of the best mods I've made to my Jeep. It has made it much more enjoyable to drive - both on and off the road.
I would do some research into what kind vehicles are available to use as a donor, weigh your costs (add at least 20% to your total for unforseen problems), make a budget, and go for it. Just MHO. E-mail me if you have any questions.
'85 CJ-7 with '94 4.3L Vortec w/4L60E tranny. Line-X inside, outside, and underneath.