Charlie,
The original GT40 used a smaller diameter bellhousing and consequently a smaller diameter bellhousing. The ground clearance issues which have been raised relate to the larger diameter bellhousing protrusing below the chassis (there are piuctures on the forum somewhere which i pressume you have seen).
I have not seen the Safir bellhousing but bearing in mind its provenance i pressume it is pretty damn correct; John Wisher who posts on the Forum also produces an original bellhousing which is cast in aluminium and is availiable in the UK and (pressumably) overseas. This bellhousing incorporates the lower bellhousing mounts so the transmission is mounted to the "horseshoe top and bottom" rather than just the top pendulum mounts which some people have observed could lead to the transmission swinging like a pendulum.
Obviously the smaller bellhousing needs a smaller flywheel and clutch assembly and in the UK these are availiable from Southern GT.
The ZF and RBT 5 (and 6) speed share mounting faces so will mount to all thes bellhousings. One point you should check with Safir is that their bellhousing is of the six bolt variety. The GT40s had a dash 0 or dash 1 gearbox which had a four bolt mounting. the dash 2 has a six bolt mounting and this is what John's Bellhousing is designed to cater for.
I had considered the 6 speed ZF but actually struggle to find any merit other than in the numbers game. The Le mans winning cars had a 4 speed and with the torque of the engine needed no more to top 200 with flexibility. Surely the 6 speed will only add weight at the rear where you lease need it, why try the ZFQ which I believe is a few pounds lighter and a modern evoloution of the ZF.
This links to a previous thread on the subject.
Bellhousing engine mount lugs
Miles