Does the pump have the high-zoot NASCAR 100 psi relief spring? If so, put the stock 60 psi spring back into it.
Ford pumps are also prone to have a leak at the pickup to pump junction, and they will attempt to pump, even show some pressure, but end up starving and breaking.
Also, there could be a problem with the distributor pisitioning in the block. Ford distributors sometimes sit in the block too deep, loading the pump drive shaft against the distributor body from the shaft stop, and the center oil pump gear. This is common when people over shim/tighten the distributor shaft stop and/or don't check if the pump shaft is binding.
Shims are available to lift the distributor a small distance, so the whole dist manishaft, driveshaft, oil pump gear doesn't do a hard stackup.
If you find the stackup problem exhists, you could carefully shorten the drive shaft, but just don't get it red hot. Best way to do it is to shim the distributor body up if you need to. Most of the time, if the distributor is staking things up, the bottom of the distributor drive gear is also ridint on the block face on the bottom of the distributor hole, and will need to have the distributor body lifted with the shimming method.