If you have a little bit of fabrication skills, you can install Pontiac Fiero seats. I got set for $50 bucks off of Craigslist. There is a guy in Florida that makes upholstery kits for the Fiero seat.. I sent him my backseat cover for a pattern and he matched it perfectly. I did have to send him the fabric that I purchased separately from the manufacturer of my seat covers, but that was pretty cheap.
MrMike (the upholstery guy, Google him) sent a DVD showing me how to install the covers. I did the job in two days.
Below is the result. The seats fit nice and low and are very comfortable. Most uninformed persons think they came with the car.
This is a pic of the mockup of the seat brackets. I used the Fiero assembly. I had a local shop make the "Z" brackets for me (They did it for free). At the end of the day, I ended up cutting the front brackets off and welding on a straight piece of steel to lower the front of the seat. Its pretty much where the stock seat is right now. The pic below was design phase 1...before I cut the front mounting brackets off of the front.
1965 Mustang convertible
331 Stroker with EFI, 355rwhp - 358 ft/lbs
Street or Track coil-over suspension