Hello everyone again,
I have posted several times. Baby steps. I purchased a 2002 - 28' Crownline a while back and the head gasket blew on the first outing. After a long and complicated ordeal that is not part of this question and is too painful to go into detail, I have a new 496 Mag standard 375hp (Purchased from a member on this forum) (It was a short block but put mostly new on it--new raw water pump, new water pump, new alternator, new coil packs, new injectors, new IAC, new cam position sensor, converted timing gear and sensor to 2004 model, re-built heads, new intake manifold, new risers and exhaust manifolds). . It is finally in, wired correctly, with the outdrive hooked up. We pulled the fuel line so as not to hydro lock it or have it fire up while we cranked it to build the oil pressure up. (We cranked for 10 seconds, let the starter rest for 30 seconds and repeated 6 times.) Then we hooked the fuel line back up, heard the fuel pump kick in when we turned the switch, and she fired right up! (for about 10 seconds). We cannot keep it running for more than about 15-30 seconds. We are not sure where to start troubleshooting.
Any advice as to where to start troubleshooting?
thank you in advance

Check fuel pressure. Key on engine off KOEO should be 42-43 psi. Engine idling will be fluctuating 38-42 psi due to vaccumn on regulator.
Access to a scan tool?

See if your mechanic has a injector cleaner,if he does I would hook up the cleaner canister at the fuel rail and turn fuel pressure up to about 48lbs and see if it will continue to run

Agree, check fuel pressure