No oil flow mystery

Helping a buddy try to get his wife's car running, and we've run into a bit of a problem.

1964 barracuda, slant 6, 1965 block with 1975 head. Just had the engine built by matco in Texas and we installed it today. Got everything buttoned up, go to crank it for break-in and we get no oil pressure, or flow to the top end.

Took off the oil filter and started the car and it shot oil to the firewall, but put the filter back on and back to no flow.

There was a plug in a lower part of the block that was missing, we put that in before we installed the car. That being said however if they missed that one in the build I would not be surprised if they missed others, but there is no oil leak anywhere so maybe an internal plug missing?

Anyone happen to have a few ideas for me to check out?