That should more than help a little. 50 to 65 is a huge change on fuel pressure. Are you seeing that disconnecting the battery for a while actually resets the fuel trims? I was not aware that would work. A scan tool or just driving it for a period of time usually is what is needed for the fuel trims to relearn/reset.
About the timing situation, where are your knock sensors located? Do you have the part numbers for them documented? The right and left should be different part numbers if you followed the Resource Thread.
Also curious, if there could be a fuel line, wiring harness, etc. that could be getting pinched, yanked, squeezed or whatever as the engine starts torquing over as load is increased? About 15 years ago when did my first 8.1 swap, I had a fuel pump lead on the back side of the engine on my K10 that would cause issues under hard acceleration as the connector got yanked as the engine torqued to the side. It wasn’t until it finally pulled apart and the fuel pump completely stopped that I finally figured out what was happening

. Oddball things like that can cause oddball issues.
Are there any odd noises coming from the engine that you’re planning to address later that you haven’t mentioned? Like a flexplate rubbing a dust cover, exhaust hitting the frame under load or anything like that? Knock sensors will pick up on that stuff. They can also pick up on chassis and exhaust vibrations when using those tiny barely cushioned competition type engine mounts as well.
I also thought I saw where they had this harness in stock instead of building a fresh one? If so, it may be worth looking at the Cam sensor terminal locations inside the connectors. The harnesses are basically the same between an LS to an 8.1 except for the cam and crank sensor terminal locations. The positions are swapped inside the connectors. Possibly, this was an LS harness that they quickly turned into an 8.1 harness and forgot to repin both connectors. If that would be the case, it would probably be on the cam sensor side as it wouldn’t even start if the crank sensor was backasswords. It will run without a cam sensor, but not as good as it should and you should have a code for cam sensor but not always depending what series of codes they turned off in the tune. It’s just one more thing to easily check off the list. I've seen many more pinout issues or pins not fully pushed into the connectors with Howell than tuning problems.
Lastly, do you have any video footage of driving or if it running? Things just don’t add up and getting 2 bad tunes from Howell would be odd. I say that but, I’m chasing my tail with Troy right now on a 0411 swapped L29 454 where I am confident he has the wrong tune in it because the Tech II says its spinning 5000 RPM at an idle and won’t accelerate. I am almost positive he has a 4x reluctor L29 tune in it instead of a 2002 24x L31 350 (that used a 0411 ECM with an actual distributor) base tune with L29 fuel and spark trims sprinkled on top of that. That’s kind of an oddball Frankenstein, but they should have their bone stock 8.1 tunes pretty well dialed in.....one would think and hope
Hopefully, I will have my 8.1 tune for you try sometime today or tomorrow. Still waiting for my buddy to let me know that he got his tuning laptop back from the computer shop.
Here is the 8.1 cam sensor pinouts
View attachment 482986