Today my wife assisted me in some measurements using a Kirkey seat to determine some critical dimensions of my chassis. I'm not too terribly afraid, though, as I've been working with the Caterham model on SketchUp. As long as that's accurate, my frame should be similarly sized.
I'm going with seats built into the chassis and adjustable pedals. With a 40" wide chassis (2" narrower than book, 1.5" wider than Lotus) and 4" tunnel that leaves 17" wide seats. Plenty!
This is a 16" wide Kirkey economy 10deg layback seat. We tilted it rearward by jamming things of specific heights under the front and took measurements based on Allan Staniforth's "Race and Rally Car Source Book"
These seats don't sit flat, and if you don't pay attention you'll find yourself like a turtle!
Vulnerable to attack!
The wife has to, of course, fit as well.