We never see any growth in diesel tanks as far north as I am. Or at least is far less prevalent. I have never seen a case. Causes a world of grief though in hotter climates apparently.

Car deserves a complete fuel system cleanout. At least more labor intensive than cost.

That said I would first run and drive the engine on a jug with the pickup and return line in it. This simply because you noticed a power improvement with the enhanced relief valve. This to me is not consistent in some ways.

You want to localize the problem of where it is originating at although the fuel system cleanout will be needed. In fact the jug may indicate it is the only problem. Still I would not want to clean the fuel tank out to find I still had a problem at this point.

Just because it is a fair amount of work and can wait. The jug test is not rocket science. This test may also indicate if the lift pump needs cleaning or kitting as well.

I trouble shoot in ways that may seem different at times. I do not think I am lazy. Instead just do not like to do a lot of work until I am satisfied it will all work out well.

I also do not expect others to do as I do. In this case it would give me a chance at evaluating any additional problems or the lack of them at the lift pump and beyond. You can never be sure of what you might run into on very old cars . Put new fuel into the jug if you go that way. Bad fuel or fuel with a lot of water in it may also be in that tank. Mind the crud.

Even just dividing things up can make things easier. Hopefully your total problems are just the tank filter and a dirty tank.

That pesky improvement with the new relief valve to me should not have occurred. Basically if the engine was starving for fuel increasing the relief valve pressure release unless the old one was totally shot should not have done anything. At the same time some effects cannot easily be accounted for.