Your seats aren't too low. Bench seat outer bolt holes are too far outward for buckets, as you already know. But they're convenient. Ideally, the outer holes should be about 1 1/2 inches further inward. Aftermarket weld-in brackets are available for inner seat mounts.
I have just converted universal bucket seats that came out of who knows what to the original bucket seats that came in a 68. My car originally came with a bench seat. I have installed the outer track of the buckets in the second set of holes to the inner side of the car. Yes, the seat rubs the seat belt IF I go too far back. Being that I am 5'11" tall, I don't need the seat the whole way back. The side strips for the seat rubs a little but nothing too much to worry about. The inner track, I made a bracket and welded to the under side of the body supporting the inner seat track more firmly than just drilling a hole in the floor pan and bolting it to the floor.
Personally, the seats are really tight to the floor and really look good sitting in the car. They are not on a tilt, and sit perfect. If will notice that the aftermarket inner track braces that you can buy from year-one have a bump on the front bracket. I solved that problem with some washers that are hidden under the bracket to position the bracket where its suppos to be.
Even though the seat isn't EXACTLY where its suppose to be, it looks and feels like its original.
If you want to know more about what I have done, feel free to e-mail me
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