IIRC, the HX35 and HY35 were available on the 5.9 B series Cummins, while the HX40 came on the much larger 8.3L C Series. I am not sure what turbo came on a 4BT from the factory, but I would expect that an HX35 would be a bit laggy on a 3.9L motor that has not been fiddled with (higher RPM, larger injectors to compensate for additional air flow, ECM programming, if required, etc)
I am interested to see what you come up with on this. I helped a mate of mine move houses over the weekend and found out that he has two 12 valve 5.9 blocks, two 47RH transmissions and a 271 transfer case. Hmmmm, that sure would solve the 4M50 power and tuning problems
With the 4M50 being 4.9L, maybe it is close enough to the B Series to work out. Something in the 250 BHP and 450 TQ range would be excellent for these trucks IMO.