I'm not aware of anything premade. You can build some adapter brackets out of a piece of flat metal and some channel iron. The flat metal I used was long and wide enough that it would bolt to the floor using the factory bench seat mounting holes once I drilled them and it was about 1/4" thick. I welded the channel on top of that AFTER I had drilled the appropriate holes in the channel for the bucket seats to mount to and centered the seats in the passenger compartment. The channel was 1 1/4 " think if I remember right. The factory seat mounting area is flat ans about 1 1/2 " lower than the area by the trans tunnel where the other side of your bucket seat will mount. The brackets I made take up the height difference and keep you from having to drill extra holes in the outside of your floor. What is left them is twisting your inner brackets or making spacers or whatever is needed there and drilling the mounting holes in your floor by the trans tunnel. I'd show pics of the brackets I made but I have gotten rid of them.
I used the Speedway mounts that Dave posted. They are universal mounts so you will need to cut and reweld them to fit. I used one of the original bench seat mounting hoes but had to drill new holes in the floor pan for the other 3. I would suggest you get an angle gauge so you can get the right front to back slope for the seat. I have pictures of my brackets after I modified them if your interested. Just PM me and Iíll send them too you. I havenít figured out how to post pictures on here without using taptalk.
I'm dealing with this right now except I believe my buckets came out of a 66 GTO, so basically the same seats you have other than upholstery pattern. I looked and could not find anything as far as a bolt in bracket either so before I cut the original floors out of the car (I'm doing a one piece replacement floor) I wanted to hash out how I was going to mount them. I'm also 6'2" and want the seats a good way back from the steering wheel so that played into this as well.
I don't have photos of this so I'll do my best to explain it. Let me say in advance that there are many ways to skin a cat when building these cars. Since my car is a late 60's style build I feel this looks period correct.
If you look at the stock GTO/Chevelle seat brackets, you'll notice that the front legs are taller than the rear ones. This appeared to be the opposite of what would be required to fit the 57's floor. Making matters worse, as the floor gets closer to the trans tunnel there is a hump. Just using the stock brackets as installed in the GTO/Chevelle's would require the seats to sit too high in the car for me to be comfortable.
After a lot of back and forth my buddy suggested we try swapping the driver seat brackets onto the passenger seat and vice versa to get closer to the shape of the 57's floor pan. What you end up with now is the tall legs in the rear and the short ones in the front. Also, your adjustment lever will now work in reverse. That got us much closer to a decent fit.
With this change made, we bolted the front inner foot of each seat directly to the floor. The front outer foot will have a small 1 1/4 inch tall piece of tubing under it which will level it out. As to the rear two (tall) legs, the distance you have the seats in relation to the dash and the angle you prefer the seatback at will determine the size of the spacer block required. Additionally, I will be using the Taiwan floors in the car and I have heard that the metal is not quite as stiff as factory. To compensate for that I am going to install a small mig welded plate onto the floor where the rear mounts will sit to provide a little additional rigidity. Hope this is of some help.