The price was right for me on those ProCar seats, so I'll continue tweaking the driver seat until it suits me. Right now I'm as low as I can go and still retain the slider mechanism. So I will try to make a stationary mount sitting right on the floor pan, while still utilizing the stock TR6 mounting holes. As for the seat back, I think at this point I will continue carving foam out. I am only removing foam where my back rests against the seat and not around the perimeter, to avoid having to cut and re-stitch the vinyl.
Thank you for that lateral thinking, Jerry. I do get stuck on a solution sometimes, leading to weeks of head scratching. Good luck with your hunt.