I was wondering about the same kind of repower for my HD11B (the original 11B, not the Series II). Displacement-wise, an 8.3L Cummins would be just right. But at 300HP, that's a fair bit more HP than the 516 Buda put out (120HP) so I'm not sure that the tranny and drivetrain could sustain it. So now I'm thinking that a 5.9L Cummins might be a better option. It started out around 160HP in 1989 Dodge Rams, so should be about right.
So far the 516Buda is still running, but it always good to be thinking ahead...
Andrew