Slant 6 head interchange: 1975 head on 1969 Block?

The difference is in the rear cam journal/bearing (fully grooved on the hydro motors to supply more oil to the rocker shaft, then from there through the rocker arms, down through the pushrods, and into the lifters). So if you are retrofitting solid-lifter engine to run hydro lifters you have to groove the rear cam journal or bearing and install the hydro rocker shaft, rockers, pushrods and lifters. But no, there's nothing about the block that impedes enough oil flow to preclude the use of hydro lifters.