Buick 225s and 231s are popular swaps into flatfenders. just estimating here but the 225s were used in Buicks and then Jeeps from the early to mid-60s to the early 70s. We have one in our CJ-3A and with the overdrive the thing will hit 80 if I really want it too... though with the loose steering and tiny brakes that doesn't really happen. I've done 65 on the interstate relatively comfortably though. Amazing power and low-end torque, especially in such a light vehicle
The 231 came out after the 225... very similar engine... slightly larger bore I think but not sure. It's actually still being used today in a few front wheel drive GM cars, though now it's called the 3800 (3.8L) and has modern fuel and ingnition systems.
If you aren't afraid of fuel injection a cool swap might be a 3.8L out of a late model camaro. I think those were the last rear-wheel drive ones. Might be overkill for an otherwise stock flatfender though.
A modern 4-cyl would also be a good idea. The iron duke is a great engine if you could find one setup for RWD.
'87 YJ Laredo
- 4.0L, AX-15, NP231, 4.10s: 3.5" BDS, 31" Muds
- Donated drive train to the YJ, Parted out the rest
- Buick 225 V6: Painless Wiring Harness, Warn Overdrive & Hubs, 2" Lift, 31" Muds