Dan, the chassis is an all-aluminium alloy semi-monocoque....hence no access to the gearbox from above. The tunnel is open on the underside (see pic.) for most of its length and the crossmember for gearbox support is fitted separately, after the engine and gearbox assembly is fitted.
Which manufacturer? My car is a PACE 289FIA. PACE hail from Arundel, just south of Brisbane, Qld, Australia. The chassis is their design, and the body is a PACE-modified Bruce Chervenak body, reinforced with carbon fibre and kevlar, and modelled on CSX2323.
To my knowledge there only two PACE 289 FIA/USRRC cars here in Australia. The other one, a USRRC replica of Ken Miles' car, is near Canberra, around 800 miles away.
Here's a view down the tunnel....
Cheers,
Glen