Yeah, to get it installed behind the TBI you generally have to loosen/remove the fuel line clamp attached down behind the engine at the top of the transmission, so that you can adjust the line position to allow for the adapter. That was my experience on the small block; figured the big block is similar.
Your pressure seems good but I wonder if the pump is working overtime to build it due to cracked/pinholed fuel line at the sender? It happens once that line gets old since the fuel is constantly attacking it. I wouldn't suggest going to the hassle of dropping the tank or lifting the bed just to check, but you might end up there anyway. A fairly easy test would be to very briefly deadhead the pump and see how much the pressure spikes. Unfortunately I don't have hard numbers on what to expect, but if it changes starting behavior immediately, you may have found the issue.
I do the deadhead test by carefully clamping a thick piece of plastic folded over the rubber section of the return line. You don't want to go nuts and damage the line. Clamp it off and watch the gauge as you start the engine, then shut it right off.
Fuel pressure regulator inside the TBI may be getting weak, or that line from pump to sender may be the suspect. Again I'm not saying you should immediately go into the tank, but it's a possibility.
FAIR WARNING this test may possibly find the weak component and send it over the edge, so you may not want to do it right now if you're depending on the truck for transportation.
BTW this is what years of sitting does to that line
this is obviously a very extreme example, but is a reminder of how harsh an environment inside the tank is. The fuel has very clearly gone super bad and turned the line to jelly... but in far less extreme examples it causes cracks and pinholes in the line and makes it hard for the pump to build sufficient pressure to start. I had one where that exact issue caused long crank times before starting.
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Richard