5.3 FI swap... acting carbish?

[JNJchev77]

Not sure that makes sense, but I'll explain...

When I fire it up it starts right up for about a second or two, then dies. I try to fire it again and it struggles a little to start, but will fire up... then dies again after about a second or so.

When it fires up and I give it gas, it will stay running but I have to keep pressure on the throttle for 20-30 seconds for it to finally idle. I haven't checked fuel pressure lately, but initially it was at like 57-58 psi IIRC. I've driven it several times and don't have problems with it bogging down at WOT or in between, so I'm not sure if it's fuel pressure, possible something else?? It's been like this from the start, just moved on from it to get some other things dialed in for testing...

Any ideas as to what the problem is?

Specs:
5.3/4l60e ( Motor had about 120K, opened up and all "looked" good )
zo6 cam, new springs, stock replacement pushrods 7.4's
Fuel Pump - Walbro 255 brand new

Any help is appreciated!!
Justin