Starting and idling problems

Update: I retried the starting with the rebuilt 5.7s after rebuild, struggled to start first go, then second go it fired and then died, if I give it some throttle it would keep running although felt a bit rough. Since I only have 5 X 6.1 injectors that I know flow I had to use 3 X 5.7s. Not ideal but has worked in the past. Sure enough it fired up and ran and idled without stalling.

So it's either a fuel pressure issue OR these 5.7s have something wrong with them.

What's the difference between the 5.7s and 6.1s pressure wise, I believe I've read 30lbs vs 34lbs? Would that be enough to cause a starting issue if pressure was an issue. So now I'd have to install a pressure gauge to check? Pity there isn't a screw on port like on some other factory fuel rails