Slant six or v8 swap for use in a truck

We have twin trucks. Mine is also an 82 D150, slant and A833OD. You don't mention what trans you have, is it the granny-low 4-speed or the OD 4-speed? If it's the OD, a far cheaper way to go is just change the rear gears. I put 235/70-15 tires on mine, they are 29" tall. I believe my rear gears are either 3.23 or 3.55 (can't remember). I'm going to change them to 3.91. That will be a livable deal around town, but drop the cruising RPM to somewhere around 2200 on the freeway.