I've been working on this car for about 8 months and was very excited to get it back to together then this happens so this has been a disappointing weekend to say the least.
You have much to learn, young grasshoppa. This is a minor speedbump.
Your car will start and run with the fuel tank on the ground, even without the evap lines hooked up. I've done it.
First thing I would do is confirm your fuel pressure is reading correctly. Jumper your pumps (key on, use a jumper to ground the - side of the pump), and see if your fuel pressure moves when the pumps kick on. If it does, good, you have a something to refer to.
I reiterate the previous question; do you have a PPRV delete? If yes, the pressure draining off after prime is perfectly normal. If not, it strongly sounds like there may be a leak at one of the joints in the tank. Have you looked in the tank?
If your fault code goes away when you hook up the stock wiring, that points to a problem in your wiring or second FPDM. Try hooking it up to the second FPDM only. Is there a fault? Then bad FPDM. If there's no fault, it's the jumper harness.
Regarding the fault code, here's an excerpt from a thread over on corral as to possible causes:
This is a fuel pump secondary ckt malfunction. Have you recently done anything to your fuel pumps?Is this code a hard fault or a continuos memory code? Your vehicle is a returnless system with a fuel pump delivery module which is mounted in the left side package tray area in the trunk.This module controls your fuel pump,it actually duty cycles the pump to maintain the pressure requested by PCM.This code is set when the FPDM detects a fuel pump secondary ckt fault then it will send a message to the PCM thru the FPM ckt indicating a failure has ocurred.The FPDM will then duty cycle the fuel pump to 75%(750 msec ON, 250 msec OFF) as long as the problem is present.Once the problem is no longer present the FPDM will return the duty cycle to 50% and the P1237 code will then be set to continuous memory. Possible causes are : Open or shorted FP PWR circuit
Open FP RTN circuit to FPDM
Open or shorted circuit in the fuel pump
Locked fuel pump rotor