Truck Repair: chevy s10/or blazer 2wd (sorry), oil pressure switch, chevy s10

Question
i bought an s10 body and chassis (no engine)
it originally had a 4cyl in it and i had a running 4.3 vortec in a rear end totalled blazer tranny too. i took the motor out and put it into the s10  the s10 was a manual so i had to change the steering column out of the blazet into the s10. that all went together well (both are 1995's). i put the high flow fuel pump into the s10 (eighth letter in on the blazer vin number was a (W).tried to start it and found out that i had to change out the wiring harness behind the dash going to the computer(yes i changed the computer too out of the blazer) tried to start, have power to guages and everything right, so go to start and the truck turns over , has spark,has compression/just ran a few weeks ago has great fuel pressure with new pump. when you turn the key to on, you hear the pump kick on for  few seconds then shut off like its supposed to.now, if i unplug the fuel pump from under the hood and turn over the truck after a few seconds it will fire and sit there and run  without a fuel pump but runs horribly. weird huh? now today is friday. i just got hired and have to start work on monday . ya delivering pizza. so i dont have a vehicle , any comments will be greatly appreciated.l

Answer
Hello Cameron,
Is the fuel pump the one that goes to the 4.3 Blazer?
If so, you say it has great fuel pressure, but it may have too much fuel pressure.
If it is throttle body injection, you want about 14 PSI at the engine while running.
If MPFI, I believe 45 PSI is what you want.
Make sure the return line isn't stopped up causing the pressure to go too high. Make sure the pressure regulator is working.

I really don't know how it is running, unless what you are disconnecting is the fuel pump relay. There is an oil pressure switch that also runs the fuel pump, in case the relay doesn't work.

What happens when it is running horribly and you reconnect the fuel pump wire under the hood? Kill it? Maybe even just connecting it momentarily.

I would really want to know the pressure...at the fuel rail.

A wild thought, is,  is there any chance you have the wires to the fuel pump reversed?
It would run backwards.
Or the LINES, puting pressure on the pressure regulator, and feeding the rail backwards? I don't know if it is possible, but a thought. Lots of strange things can happen in a big swap like that.

Van