I wonder if it will fit also. I didn't know there was a difference between the 81 and later 258 blocks.
I have the 4.0 HO head (7120 cast) and see about 15 mpg with 4.10 gears and 31" tires with T5 trans. I drive with a pretty heavy foot so I'm sure I could squeeze a couple more mpg but it wouldn't be as fun. The head mod ain't for mileage but some poeple see some good gains when running a FI setup.
1986 CJ7, 4.2 w/4.0 head, TFI-HEI hybrid ignition, Clifford manifold w/Holley 390 w/cold air intake, OBA, 4.5" lift, Woody CV shaft and Tattons in front, 4.10 gears - lunchbox in front, Truetrac in the back, twin-sticked, blower upgrade for running topless, trying to keep it simple.
It's just a Jeep, and if you don't wheel it once in a while, it's not even that.