Welcome Tim
What's the secret to maintaing your girlish figure?
Probably the best idea is to sit in a few and notice where the back and bottom of the seat is in relation to the hard limits imposed on you by the back and floor of the passenger compartment.
Below is a pic of me @ 140+ kg. (am 6' - 6'1") in my RMC (local to you)
There is a bigger version in the gallery (click on the thumbnail - then on the picture that is shown here and you'll get the bigger version - you can also be impressed by the reflections in the paint job.)
Later I lowered the seat and moved it back - this made a big improvement .
Good luck with your build. There should be no excuses for 'projects being put on hold' now the mining tax has been forgone
fyi
http://members.iinet.net.au/~zamia22/cobra/index.html
LoBelly