slacktavist,
I asked a similar question prior to my build, have a look here:
http://www.haynes.co.uk/forums/showthread.php?t=3745
for a few more takes on your question.
(Note: Contary to my last comment on the above thread, I have actually added some length)
I am building my chassis from scratch (i.e. not a pre-cut chassis pack) and had the opportunity to lengthen the chassis if need be. If you buy a pre-cut chassis pack you would need to speak to the supplier who may accommodated you needs for customisation. Remembering that any customisation you carry out may limit you in buying further parts, e.g. GRP side panels.
I bought my seats prior to cutting any material and when I laid out the lower rails put the seat in position and hence could gauge the position of my feet with respect to the pedal box. As a result I have added a few more inches to the cockpit length. It probably would have been OK, but better to ere on the safe side i.e. too long and move the seat forward on its runners rather than spend hundreds of hours on the build and probably still fit but not quite as comfortable as it could have been.
I did have a 'test fit' in Tilly's car at the Stoneleigh show. I did fit fine, but as a previous contributor has mentioned depending on your choice of seats several inches can be taken off the leg room.
Hope that helps.
Welcome to the forum and good luck with your built.