Keith,
The changing of gearboxes from one type to another is a VERY LARGE job and it would be safest to say that NOTHING FITS !!!!!!
New mounts, adaptor plates, clutches, release and spiggot bearings, starter motors and definately gearchanges would ALL have to be redesigned and fitted. A lot of time could be saved with purchasing some parts from a manufacturer but the clearance issue might cause problems.
I would suggest that you keep the renault box and increase its strength. Malcome in England is racing his GTD with one of these boxes and I have sen him give it some serious stick at Brighton. The change over shaft that GT40NZ offer looks very good quality and I would be starting there.
Best wishes,
Robert