I would guess at a 6 speed box off a supra (or a T56) with an Omega auto bell housing plus an adaptor plate between the bell housing and box.
now I am really impressed - I have a Borg-Warner T5 sat in the shed ready to go in the track-day car using just that - an auto bellhousing with a welded-on rear plate.
the options as I see it are :-
use a standard gearbox and have done with it
use a Ford type 9 (5-speed) gearbox as per Kev's post
use a Caterham gearbox which is basically a 6-speed type 9
Toyota 6-speed will need a bespoke bellhousing
T56 will need a bespoke bellhousing and is damn near as heavy as the rest of the car ( I should know, I've just rebuilt one )
The ZF box will need a bespoke bellhousing
.... and any bespoke bellhousing I'd trust would be in the order of £2K
there does also come a point where you'll spend more time changing gear than putting down power in the gear.
My previous track-day Omega had a 2-litre gearbox in it from the moment I took the auto out.