Someone needs to start re-casting 340 blocks

???? The Cleveland engine was the performance alternative to the Windsor period. If You mean swapping a crank w/Cleveland size bearings, & spacer/shim shells, yes, there is less friction. The 360 was designed ground-up as a cast crank unit, & in typical over-engineering fashion, Mopar probably made the mains larger to maintain crankpin/main overlap.
I'd love to see back to back dyno testing of different size main bearings in automotive engines. I just bet there wouldn't be much difference.