I have seen on all GM rear caps SBC & BBC we have found then to be restricted because of the casting hangs in past the oil port of the oil pump and we have seen some that have been block 60%
Take a oil pump cut it off like the one on the right.
The cap on the left has been opened up to match the oil pump.
The cap in the middle had been marked with a marker showing how much has to be removed.
The cap on the right you can look in the oil port and see the casting blocking the port.
Even on the after market blocks they some times have to be touched up.
