Clois,
Was this block ever modified for use with a solid lifer cam? If so, you may not be getting proper
oil flow to you lifters unless the oiling system mods were removed/changed. It may be useful to find an old stamped valve cover and cut the top of it out. Install it on one bank and see what kind of
oil flow you end up with in the top end.
A more common problem with the FE's is too much
oil in the top end, hence the use of restrictors to limit the amount of oil going into the head. The open-top valve cover will also identify any drainage problems associated with too much oil.
Have you checked bleed down of your lifters to ensure that they are pumped up? I guess it is possible that you are essentially operating as a solid lift cam if there is no oil flow.
Troubleshooting is often no more than a process of elimination. Proving all of the above to be OK will narrow down the potential problems. The advice above about the stands and oiling should also be checked as I hear that is a common mistake when putting together an FE - kinda like head gaskets.
Good luck,
Gary