There are always pistons going the opposite way, so not really much of an issue. Crankcase vacuum is more for ring stability in higher revving engines. Red Diamonds ain't. It may add a couple horses at the 4000 rpm or so point.
A 6.7L Cummins isn't a bad comparison. A lot will depend on camshaft and turbo selection. It could cover a vast range of character.
A DT466 may be more in line with the sheer mass. I plan to measure the next one I run across, but think they are not far removed from the Red gasser. Up side of that would be the availability of a billet aluminum head... Gas/E85 converted Cummins pickup engines are popping up more often. CNG/spark ignition models have been in commercial use for a while.
RD450 is 4-3/8 x 5
RD501 is 4-1/2 x 5-1/4 and has the crank lower in the block, farther from the cam, uses a unique set of timing gears from the 318-361-372-401-406-450
Stock rod journals are 2.750 Lots of weight and bearing drag.
BBC rod journal is 2.200
Offset grinding a 450 crank will allow the extra 1/2 inch, but require custom rods. It will take several pounds off the crank, and allow several more to be removed from counterweights. I'd bet 20 could come off, maybe more, after cutting a couple pounds per piston as well.
RD pistons are massively tall so even leaving the crank alone, the RD rods are too short and not very strong anyway.
Rods are 9.00 long, stock with 2.4 or so pin height.
Pin height is an inch closer to deck in a BBC piston, 1.27-1.65 or so commonly
Custom pistons could be made tall enough, but the rods are weak and big ends huge.
My combo, with roughly 1.270 CH pistons, and .250 more crank throw, calls for 9.875 or so connecting rods. YMMV