41 pick up fuel tank sending unit

[41 Ford PU]

I have a 41 pickup. The fuel tank was leaking gas when it was full. I suspected the sending unit because the tank looked to be new. After quite a bit of aggravation, I finally got the tank out and found that it is a Drake tank which is why I had to disconnect the bed and raise it up to get the tank out out.

Now, I found that one of the sending unit screws was stripped and wouldn't
tighten enough to seal the tank. After trying to replace that screw, it simply stripped again. So, I replaced all 6 screws with 10-32 x 3/8" screws (they were only 1/4" to 9/32" long). I added an extra flat washer to take up the additional 1/16" of screw length. It has a cork sealing washer. It still leaked from around the sending unit seal. I suspect that the cork washer is shot.

Here's the question: Is the cork or the neoprene washer better? Also, is there a sealer that I can use as an additional sealing compound?

Thanks for your input.
Mike