Simon,
Good luck !
The reply above is correct, I would add buy the right parts first time- a lot of GTD & Tornado parts can be used, for example Tornado bellhousing or GTD adaptor plate. Use an AP Racing clutch if you go the adaptor plate route. Don't scrimp on brakes. Granada/Scorpio brakes work on the road but will not withstand track use. I use 600lb back springs & 450lb front with custom-made AVO dampers. The standard type c chassis will take a 289 or 302 without modification. Buy your front & rear windscreens when you are fitting the body as the reveals will need to be modified. The wheel arches on the nearside will need modifying too as the body is not symmetrical & will foul the wheels. Similarly, the left hand filler cap must be raised. Watch out for assymmetric rear suspension mounts- some chasses were incorrectly measured & the top trailing arm bracket was welded in the wrong place. It should be moved to match both sides. Most instructions reside in 'Fortification' magazine, so it's worth joining the GT40 Enthusiasts Club, it was the best £25 I ever spent. I believe the Type C has the best chassis, but it is the most difficult replica to build.
Good Luck,
Tony Jasper
(Type C with Ford289/R21T gearbox)