Swapping an LS into a 50 GMC fuel questions

[suburban99]

I am swapping a 2004 4.8/4L60E into my 50 GMC on an s10 frame.
I bought the motor and tranny for $1000 and when I removed the vortec cover the intake had a large hole in it near the MAP sensor on the top. This manifold is returnless, and DBW.

I obtained another manifold off a friend that is DBC and swapped the DBW throttle body onto it and ran fuel and return as it has a return line. I repin my own harness and delete VATS etc with HPTUNERS. I have done a few of these swaps.

Truck cranks and starts for about 2 seconds but does not run. Fuel pressure is about 50PSI and falls off quick when pump shuts off (after the 2 second run form the ecm). Fuel relay has power during crank and run.

Question 1 Are the manifolds interchangeable or should I grab another DBW instead of the DBC.

Question 2 Why would fuel pressure fall off so fast after pump stops? Is their a check valve I am missing or is the fuel regulator leaking? Leaky injector?

Question 3 If I have the Tach module wired correctly should the throttle body flapper move with key on and pedal pushed?

Thanks.