I've heard the center seat from mini vans are narrow enough. I got a Glide Engineering frame and foam then had it done in original materials. I also moved it back 5".
Now I have to make new side panels because Henry saved 2 cents per car by not having the passenger compartment side panels meet the rumble seat ones......
You'll get a lot of extra noise in the interior from the trunk/rumble seat area. I think early Mustang seats will work. But IMHO buckets and/or headrests just don't look right.....