Is a Mongrel Jeep build thread welcome?
I think if it were for mail work I'd be looking at either the 2.5L/OD setup from a Dakota or the 4cyl Cummins Turbo deisel from a HD truck (think Cummins turbo, minus 2 cyl) which I'm told is a fairly common Jeep swap and the parts are readilly available. I think a V8 might blow the fuel allowance.