I am not a booka, please. I'm a walking 1200 encyclopedia of dubious facts.

First make sure it really is going all the way back. The stock rails for 1200 sedan go way back. I'm 6-1" and don't have it all the way back.

next if you still want it to go back further, I reckon you should move the seat back on the rails. Use an adapter plate. Which is probably illegal in your state.

in any case, read about rails here: Datsun1200.com Seats article