Seems most carbs I've dealt with over the years required 5 to 7 psi of pressure. On our off road builds we would use a pressure regulator and dial that pressure down to around 3 psi in most cases. The key was to lower pressure to help prevent flooding out the engine when the float gets to bouncing... while not starving the engine while on WOT runs.
I assume you tested fuel pressure before your pressure regulator? Speaking of that regulator... I had quit using that style regulator for the most part on our stuff and switched to the diaphragm style regulators. I had some issues with your style maintaining consistent pressure and providing adequate flow.
Float level and drop play heavy into all of this too. Seems last time I asked you never checked it because you were happy with the drivability. If you haven't check... might be time to do it.
For flow volume... I usually did the un-scientific pop bottle test. Your pump should about fill a pop bottle in about 10 seconds. Try doing it in front of the pressure regulator and again after the regulator. Wouldn't surprise me if that regulator was causing some volume issues.
I flat out deleted the regulator when I swapped the fuel pump last summer and it made no difference. I thought 3psi for a new pump was kinda low but it seemed to run the same. When I first did the V8 swap I had a cheap Mr Gasket pump on the frame, it had been losing pressure (it was probably down to like 4-5 when I did the 5 speed swap) so I swapped in the '86 in take pump last summer to see if I could get some psi back and to replace my flaky fuel sender.
If I shift below like 3500rpm (like I normally do) driveabilitly is great.
Ok, story time.
Way back when I V8 swapped the truck the it had a C5 3 speed automatic. Ran great, no vibe at all.
Then I 5 speed swapped it and noticed this weird vibration at around 3500 under hard acceleration. Power kind of flatlines too.
So I redid the rear suspension (which needed done) to no effect.
Driveline angle was close, like 3* off so it wasn't perfect. The distance between the outputshaft and the mounting base of the M5OD compared to the C5 is a little taller. Swapped the transmission crossmember with one for an explorer last summer and got it down to 1*. No effect.
Well that is weird, maybe I need traction bars? Gopro did show a little spring wrap but it doesn't really look like a whole lot. I actually bugged James Duff asking if they could piece together a kit that would fit a first gen with a 31 spline 8.8 but they never replied.
Well last month I was driving in really wacky weather, tornado sirens when I left town and the whole bit. I was trying to maintain 55mph in fifth and as I kept feeding it more onions I felt the secondaries in the pedal, just to take it easy on the truck I grabbed fourth. Guessing I was bucking a 50mph headwind (so the truck would be plowing air like it was going over 100)
And there was my vibration, constantly cruising in fourth.
So unless the truck was "pushing" hard enough to be wrapping the explorer leaf springs at 55mph cruising... well I get to thinking maybe it isn't driveline/suspension related? Most of the time I had the 3spd I lived in town, never really drove it far or fast. Did the automatic just mask the possible fuel issue by not letting it wind out? I didn't pay good enough attention at the time but I don't think it shifted that high and I never ran in any tornado weather in it to load the truck like I did last month.
Float height right on at 7/16". Tried playing with the tune and throw a little more fuel at it last summer which didn't gain anything, the next notch up on the chart (both over and up) was too rich. I have tried to push thru the vibration and get into the secondaries a couple weeks ago and while I could feel the secondaries in the pedal nothing really happens...
Its been too cold to do much since the great tornado race to do much, aside from a spell a couple weeks ago when my wife killed a tire on her car and I did the secondary test while I was dd'ing it I haven't really done much at all with the truck.