88 Suburban Problems Starting...

[AWEGS72]

I have a 88 Suburban,5.7L,automatic,4x4 3/4 ton. I blew a valve seat in my old motor and replaced it with a good running used 350. I bought it from a place i've been doing business with for a long time,and they ran tested it before they shipped it to me. They left the distributor in,and all i did was switch a few sensors,and water pump,exhaust manifolds over. I dropped it in,hooked everything up,turned the ignition and it turned over but will not start. It sounds almost like it wants to start,but then it backfires through the TBI. THe first thing I thought was timing,but I DID NOT touch the distributor and the vehicle it came out of was running fine the was it was sitting. I was told from one person that it could be poor fuel pressure,and to shoot some carb cleaner down one of the injector holes on the TBI while somebody is turning it over. Any other thoughts what it could be?

Thanks
Andy