516

Has anyone put a 516 in a d21 curious as to what all had to be done for it to fit

I imagine Mark Gettinger has done one or more. For a puller, you'd have to have a steel flywheel made anyway, so that's easy. The starter to flywheel/ring gear dimension and then maybe rework the D-21 engine backplate to fit the 516 block would solve that issue, and the bolting up to the bell housing.

A basic 426 intercooled 220 HP engine weighs 1465 lbs.  A basic 516 intercooled 270 HP engine weighs 2360 lbs. The 516 family engine (also made in a 4 cylinder 344 cube version) was a carry over from the Buda company days, probably a mid/late 1940's initial design. The 426 was an all new modern Allis-Chalmers design from the early 1960's (as also was the 301/200 family). It is a larger heavier engine design per cubic inches of displacement. If it compared to a 426 design for weight, it should be about 1800 lbs, not 2360 lbs.  The 516's physical size and weight actually compares to a Cummins L-10 (600 cubes) or M-11 (660 cubes) which weigh 2100-2200 lbs.