I did buy the new Corbeau classic seat, and it went in pretty quickly, but not without some drama. 

I guessed that the custom frame for the older GTA lowback would fit the new seat, and I was right, it bolted right on. Getting the frame out of Charlotte's cockpit was a chore, and once out, the bolts, which had been fastened with nuts, lock washers and large fender washers, were welded onto the frame, so only the fixing nuts would need to be turned from under the car (the frame bolts directly to the floor). 

All went well, and the new seat was installed. BUT, driving it home from the shop, I realized that, even though the dimensions were almost identical to the GTA, the new seat positioned me too close to the wheel and pedals, and worse, way too high in the car. It was driveable, but hardly comfortable. 

So Charlotte went back in the next day, and all the work done had to be undone. While driving to the shop, I realized that bolting it to the floor without any frame was probably the only solution, and Steve Mackay of Mackay's Garage agreed. Out came the seat, off came the frame; a cardboard template was made to indicate the position of the four holes in the seat bottom; out came the drill and four matching holes were cut in the floorpan. Back in went the seat, the machine screws were tightened up, and that was that. Now it fits like a charm, and looks great.