Oldsmobile solid roller lifters have been used in these engines before, but the link bar length still isn't 100% correct for the 6 cylinder, and its lifter body length is not correct either, and I have seen instances where they wont work at all because the link bar location is too far down on the lifter body, the hydraulic version is still a totally different story and whole new can of worms that will open up. The best solution is to find and install a lifter made for these engines to begin with. The Iron Duke of the late 1980's had factory hydraulic rollers, thats where I would start searching.
The solid roller lifters made specific for these engines by Crane and others worked 100% of the time, even with small base circle cams, etc... they were just never made in the hydraulic roller style unfortunately.
