Hey Scully,
I am using 1200 rails with some additional flat bar lengths welded to the top of the rails to extend out to reach the bolt holes under the seat which are a little farther spread than the stock hole locations. Yet to put it all together though.

The 1200 rails bolt from the ends of the car inward so parallel to the transmission tunnel where as the bolts in the 1000 (in the 2dr sedan) bolt from the top down into the floor (yikes!). I am confident it is just a matter of drilling new holes into the 1200 rails to bolt through to the framework. If I can I'll also put the other bolts through.
Then with the additional extension plates welded to the rails the seat will be able to be bolted on. I'm also leaving the ends of the 1200 rails in place so they actually fold over the framework. I'm not too keen on having the bolts (holding the seat down) running perpendicular to the direction of travel/driving, I reckon the seat will come right off with a good jolt.
Obviously it'd make things difficult for people to get into the back but I figured Cobra Classics do that anyway. For the time being this'll be how I'm doing it.