I have had my 70 Roadrunner upside down on a rotisserie doing the torque boxes, frame connectors, etc. The last thing to do is on the rear of the rear inner wheelwells. There is rust through on each side in one area. I'm going to cut that out and put in a patch. These inners were replaced in the past by a shop for the previous owner. They were never welded to the lip but rather sort of poorly crimped in place. Hence the rust through.
Once I put in the patches I need to seal up this void. I'm looking for a good way to do this. Keep in mind that the car was completely repainted and I will not be doing that except for the bottom. So I either need to use some sort of gap sealer or possibly use a fiberglass laminate like we did in high school (40 years!).
Does anyone have a product or procedure in mind that may fill this bill?

Thanks for any info