Steel case celica is now as rare a T series so thats out.
Removable RWD bellhousing gearboxes are hard to come by now.
The only one still in production and attainable is the Toyota Hilux rwd
and Hiace (side shift) however ratios are atrocious.
The rest are not so good but better than 4 speeds:
(in order of most commonly found and available)
Mitsubishi sigma rwd Astron matched transmission BW only not jap box
http://www.sigma-galant.com/viewtopic.php?f=42&t=3569This one would be the best as its avail in the GJ sigma and the model
is Borg-Warner - Model 0507. The bellhousing is close to the auto
A series bellhousing for matching up to.
BMW getrag (2 kinds one removable bellhousing differences in my earlier threads)
Ford Falcon T5 (sought after by all bogons both holden and ford fans)
W series (pinched and sold before going out to wreckers as some Arabs and other
africans dealers have contract with wreckers) however available new from Rollin
Imports and some other Jap auto suppliers.
T50 and steel case W50 (Toyota fans, Oz clubbies, Morris, holden fans and Dellows kit owners of other kinds so havent seen these boxes in wreckrs for at least 4 years now and Im at 2-4 different leading wreckers a week)
South Africa is the other option but apparently expensive there too and sought after.
Final choice for road: Borg warner from Sigmas.
Final choice for road and track: Getrag, W series
Unitary Cut and shut RWD options: 1. Mazda 323, 626, 929, B series utes RWD (929 is considered close ratio as it has the best ratios by far and light equivalent to T50 but stronger), Nissan Stumpy 5 speed (heavy but common as bleached whores), Sigma and Gemini Jap boxes (As heavy as Stumpy and as common , better one is the Isuzu Gemini/Rodeo as its based on the W series or Aisan)
Hope that covers most questions. Ive been thinking of this for many years of slumming wrecking yards and apart from the super rare "race close ratio" triumph post 1975 dolomite
gearbox the rest are above.