I had an 80 J 10 Jeep pickup. Completely different critter then a Commando, but I put a 318 in it. It already had an AMC 727 (that didn't bolt up to the 318). I had to put a Mopar 727 in it to match the 318. Then I hung a snow plow on the front end. I didn't keep it very long, the 318 motor along with the 7' plow (plow and brackets came off a J 10) made the 80 nose heavy. It was pretty worthless! I got caught in a minor snow (really, just barely covered the road, maybe a 1/2") but I had to put it in 4x4 just to get up a hill. I pulled the plow, and sold the J 10 off, bought a "real" truck to mount the plow on.

That Commando was designed to have the inline 6, or according to someone above, A gm V6 or an AMC V8 (both pretty light), a 3.9 might work if you can still find one. A 72 Jeep Commando was built nearly 15 years before Jeep became a Mopar, Don't assume a Mopar drive train swap is going to be easy.

Depending on how new you want to go drive train wise, I might consider a modern V6, or maybe even a modern 4 cylinder (turbo?). Both are light weight and probably have more power then the original inline 6, and would probably be an easier swap, space wise.