Mine was not rusted so this worked. I made it out of 1/8" x 1" steel flat from a hardware store. The new O-ring was also to large so I reused the old one. This was >15 years ago. I probably used a crescent wrench or vise grips to turn it.

While it may not be of comfort to you I will confess I have never replaced the old gasket/Oring on that application. I've changed out the intank pumps at least six times over the last thirty years and the old ones have gone back into service.

The bung arrangement compresses the old gasket more as you rotate the "keeper". I will add that I always clean them up and apply silicone plumbing grease to facilitate a smooth and tight assembly.

Randy

"I'm about to take it all apart again and install the original sender seal"
Try the old seal (large o-ring). If it goes on tight, use it.

The tool just helps turn the nut. I don't think it would help much in compressing the o-ring seal.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

I have the tool - 28211. Shipping involved.

However, after I bought it, I found I could EASILY remove the bung by simply driving it counterclockwise with light taps from a screw driver and hammer. Have you tried unsuccessfully? My car has zero corrosion - sometimes that can be an issue, although if badly rusted, the tool won't help much.
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82 242-6.2L; '17 Mazda3; '16 Crosstrek