Finally decided to go with Sparco Sprint V drivers seat as narrower across shoulders than the Cobra Monaco S seats I fitted initially
The passenger seat is a Mamba L7 cub narrow seat as used for Caterham / Westfield etc. It does hold you in reasonably but wouldn't suit anyone of "strong" build!
I am 6' 3" and everything just fits - reasonable leg length, reasonable steering wheel reach and head not quite touching roof, with adjustable runners fitted.