Sounds like what mine did. I went through everything that you have except for the mechanic (dont trust most of them).I had a fuel line on the back of the engine get a hole rubbed in it. I swapped out both of the rubber lines and was good to go. If you do this make sure to mark which hoses hook to which lines.

 

Hey thanks you guys I got it fixed!
Everyone was right in the first place, it was a leak in fuel line from tank to LP.
SO sorry I didn't reply earlier, actually fixed it last sunday then had to get right back to work, trying to make up for last week.
How it went down...
So originally I replaced filter, lift pump, did what I thought was a thorough check on the lines, etc. Cracked injectors, overflow valve, etc. checking pressure, presence of fuel, etc. Dropped tank, cleaned screen out w/ toothbrush, cleaned everything you could clean, still wouldn't crank for more than 3 secs.
Had a helper hold 100 psi air hose in tank filler tube, sealed off w/ rags to pressurize tank. as he did this I tried to bleed off air through bleeder valve. It got quiet in neighborhood that evening, and I heard something new. A "hissing" sound. I looked over everything and found that the hissing was coming from my fuel supply line, not return, right near left front wheel well. It seems that during one of my many tranny rebuilds, tranny tech had bound the 2 lines together with plastic wire tie to frame.. Every time I hit a bump over time, that had worn a tiny little pinhole right there in fuel line.
Cut & Spliced it with fuel hose for now until I can find new braided line. Fired up immediately and now RUNS LIKE A DREAM. So much more power and pedal response now with everything clean and Lucased.
ALL YOU GUYS (GALS) WERE SO AWESOME AND CARING ABOUT MY PROBLEM AS ALWAYS THANKS SO MUCH AND SOMEDAY I HOPE TO HELP OTHERS HERE WITH THE STUFF YOU'VE TAUGHT ME ABOUT. GOD BLESS!