Hard time staying running

[98 tall boy]

I have a 98 Z71 that is lifted. Just rebuilt a 14 bolt rear end for it and changed gears to 373. Changed out the front Axle to 373 as well. During the down time I started the truck and let it run regularly. Last Thursday was driving the truck and it died then it would start run for about 1 second then die. I thought this was a fuel problem but my fuel pressure gauge on it and found it to be at the low end of the spec. So i changed it out the fuel pump with a new fuel filter. the pressure is fine now but the truck is still doing the same thing (starts and runs for about 1 second then shuts off). I began to think maybe I had weak spark or some weird Ignition problem so I put a new ICM(Ignition Control Module) and coil in it. This did not change the problem. I have check the distributor and it looks to be good (it is new only have 2,000 miles on it). Any one have any ideas. I hat to think the ECM.