Caterpillar C7 ACERT in a 2005 Freightliner M2. Engine was using oil and low on power. Owner wanted new engine. Removed

Customer: Caterpillar C7 ACERT in a 2005 Freightliner M2. Engine was using oil and low on power. Owner wanted new engine. Removed engine and installed reman long block. Had to transfer oil pan and main oil pump, rocker arms and lifters, injectors and all external equipment (HEUI pump, air compressor, etc.) Installed all new o-rings on existing injectors. Owner claims that he had installed new injectors not long ago, cannot verify that other than his word. The o-rings on the injectors were in bad shape, especially the top one on the high pressure oil annulus.
Engine will not start. HEUI oil pressure varies between 1500 and 2500 psi while cranking (mechanical gage installed). Inductive voltage tester shows voltage on injector harness on output side of connector. Can hear the injectors exercise when switch the key on.
Have checked the switched power at the P1 connector (pin 70); good. Checked B+ on P1 pins 48, 52, 53 and 55; good. Have checked grounds on P1 pins 63, 65, 67 and 69; good. Have checked speed sensor resistance through P2 pins 48 and 49, 58 and 59; good. Scanner (not Cat ET) shows rpm signal when cranking 150 - 200 rpm. Have power to the IAPCV when cranking. Don't see any big oil leaks from injector bores or spill ports while cranking.
Installed mechanical fuel pressure gage output side of filter. Don't have the quick connect gage, so I made a fitting to install a standard fuel pressure gage. Fuel pressure was very low while cranking, about 5 psi. Cat book says I need 25 psi to start. Replaced the regulator at back of head. No difference. Replaced fuel transfer pump. No real difference. I can get 10 psi now.
I did a bench test on both transfer pumps. Both seem to flow similar amounts of fuel. Not as much as would expect. Ran with a drill at about 350 rpm which is close to what the pump speed will be when cranking. 50:36 gear ratio between crank and pump. Checked fuel filter (new with engine install); good. Checked fuel filter base; good. Checked fuel lines between pump and head; good. Connected a clear fuel tube to pump inlet and drawing fuel from a 5 gallon can. It moves fuel, but not very well. Can only make about 12 psi.
After cranking, let sit for about 3 minutes, then see air bubbles start appearing in the clear supply line coming out of the transfer pump. It must be drawing air from somewhere. All fittings are tight.
Beginning to suspect relief valve stuck in new transfer pump. Unlikely, but I have a relief valve stuck in a brand new oil pump on a GM 6.5 Diesel a while back that resulted in low oil pressure. Opened it back up and found a burr in the bore for the valve -- on a brand new pump.
Does this sound feasible?