Crap. I did get the engine numbers wrong.
They are:
Stbd ; OF602868
Port ; OF764092

The injectors were (supposedly) removed and cleaned before the boat went into the lake. Doesn't mean they couldn't have been fouled by the new batch of gas with any slag from the tank.
I have considered that crap and varnish are in the tank. The process to clean it / have it cleaned would be very difficult. It's hard to access and I don't know of any company nearby that can / would do it. I'm in Dallas - not Miami or Newport...
I just this week changed all fuel filters. Cut them open, they had very very little grit or contaminants. The water separator is 10 micron. Wouldn't a ten micron filter catch anything that would foul the system components?

That said, clogged / dirty injectors wouldn't cause lowered pressure in the fuel manifold. Poor running - yes. It is now on my short list to do, but perfect injectors won't fix low fuel pressure problems.

Codes were checked last year. No codes.

I had not considered the knock sensor. Would it throw a code? Does it lower fuel pressure as part of it's function? Does it re-set when the ignition is switched off?

Someone mentioned this small screen in the VST. I know it is the last restriction before the injectors... It was removed and cleaned as part of last years trouble shooting... I read somewhere to just remove it with the ethanol gas or it would cause problems. Later I think I read it's 10 micron so the same filtration as the water separators. Thoughts? How difficult is it to access and clean / replace..?

The gauge is new from Harbor Freight. New gauge, one hose... what could be wrong? The running pressure was (always, exactly,) 30.5 psi. Not 20s...

I'm sure BUP is a great guy (sorry, person...), but a marine mechanic somewhere in Texas is no help. If he is in the Dallas area, I'd love to talk. FYI, the marinas here charge outside mechanics a 20% upcharge so many won't work offsite. Can't fathom how that's legal, but you can imagine my frustration. I would be WAY happy to pay someone with the right tools and knowledge to trouble shoot this issue. But I won't pay 20% extra to have someone guess and throw parts at a problem and hope...

Not to criticize - but I'm surprised only two jumped on the pump as a problem. The low pressure is definitive. The knock sensor can be definitively be checked - so that I can verify. Slag in the tank? Air leaks in the fuel lines (that appear perfect and new)? I am very reluctant to throw time and $$$$ at fixes without knowing they are fixes. I understand boating isn't cheap. I work as a commercial pilot, so to guess and hope isn't part of my DNA....

I guess I was hoping someone would chime in that had early 502s, had seen fuel pressure of 30.5 and fixed it by fixing / replacing XXXX...
I do appreciate the inputs guys.

SN's check out 502 with VST

MEFI-1 is not that good. It does have a code 43 which is knock circuit failure
MEFI-2 tad better with code 43 meaning knock detected

Will a MEFI-1 remember the code, mine didn't. I was out having fun with the same motor as yours, many years ago. Went to take off again and could not get on plane. Could only get around 3200 rpm, motor didn't sound bad just no power. Longer story, but the knock sensor detected a knock because I laid the wire along side the plug wires because it looked neater. SInce then I have repowered again so VST is on a shelf in a box

If a knock is detected the timing is retarded

I asked about the fuel pressure gauge because HF not being known for quality equipment, I'm thinking your pressure is ok, the gauge is just off. The hint was because pressures are the same between both motors but only one having issues

I asked about if new wires were installed because the guy may have laid the wire along side them. Might have even done it just doing plugs to be nice (sort of)