It is a real PITA thing. " I feel your pain."
For the fuel pump, mounting height isn't going to matter because the height of the fuel connection coming out of the tank calls the shot. It's already high and the pump has to lift fuel out of the tank just to get over that hump.
My EFP is mounted low actually below the bottom of the tank on the chassis cross member and it doesn't matter a hill of beans.
SOME have gone to using the tank drain plug as the fuel line attachment point. It can be problematic as well because first you have to get the plug out without destroying the tank.
The tank has a threaded insert attached to the sheetmetal of the tank and a few have torn the tank itself trying to get the plug out.
The other thing is that the plug is SO LOW that you need to use a very tight fitting 90° fitting which MAY be subject to road hazards that originally you wouldn't need to worry about.
It's also a not very common metric size. I think from memory that it is a M22-1.0? That has VERY limited alternatives to use. In fact maybe even one or two that you may have to get from someone like Pegasus Racing?
BEFORE you pull out the sender AGAIN, you could try taking a small can of gas, disconnecting the fuel line from the tank and using that as a test to see if everything works then? If it does, it points even more so to the in tank pickup. If it has the same issues, don't even bother taking the sender out again. The problem is highly likely elsewhere.