And I posted in your other thread, but I'll repost here. Your S1 O2s are fine voltage wise, they jump around when in an open loop. The B2S2 seemed high and may be bad, but that won't affect fuel trim. What it COULD be telling you is that you have a clogged cat, which WILL affect driveability...
I still say that trying the 350 FPR may be your best bet. Mine required a little throttle to start until I swapped it out. I'm really not sure why they list a 3 bar regulator for the 454 (GP Sorensen #800-329), when the spec is 4 bar, but they do (at AutoZone, at least). The 350 reg is 4 bar (GP...
S1 voltage is supposed to jump all over, its called an open loop. S2 is only for cat monitoring, so it has no effect on driveability, but it looks like your B2S2 is bad. Timing looks good. IAT seems pretty high.
As long as your lines, etc are sealed. That's what I mentioned earlier. The line pre-regulator will have considerably more pressure, I believe the free-flow pressure is above 100PSI
Could be as simple as a loose line that is only losing pressure and no noticeable fluid. It's possible, I've been there. The first time I ran into it was after doing the spider on the 350, it drove me nuts. I could smell fuel, it wouldn't start right away, and it idled rough. I finally just...
The FPR and lines right at the rail is a good start, it's easy to get that FPR not fully seated. Did you put the snap ring back in? Did you install the injectors with the orings dry or did you lube them a little?
Double check all of your lines, make sure they're tight. Even if it's not puddling, it could be bleeding pressure and causing a hard start, had that issue a couple times with small blocks I did spiders on. They'd take awhile to start and run rough