Here is something i found on it... kinda what i was looking for... not exactly, but i thought id share it with the forum... and if any experts out there disagree or would choose a dif route please do share..
Automatic –Manual conversion
“Apparently” it’s an insane thing to want to do as the automatics are “apparently” worth more… but I wanted a manual... and my old gearbox had died... so this is what I did…
First things first, there are 2 types of manual mk3 Supra gearbox, the W58, and the R154.
The W58 is found on the MA-70 3.0 N/A Supras, and for some reason the GA-70 2.0 twin turbo Supras (if you can find a manual one!) it is a very smooth shifting gearbox, and was still in production in the MKIV Supra The R154 was found on the MA-70 3.0 turbo Supras, and the JZA-70 2.5 twin turbo models, it’s a stronger gearbox, with a much more heavy duty clutch. The clutch setup is different from most in that the pressure plate is bolted to the flywheel AFTER the bell housing has been mounted on the engine block, via 2 small service flaps.
Below: R154 clutch (left) W58 clutch (right)
For my 1990 N/A supra converted to turbo, I chose the R154.
(W58 pictured)
What you’ll need for R154 conversion (supra turbo manual)
• R154 gearbox. (Make sure the dealers know what they are talking about I had the wrong gearbox sent to me, and it held me back MONTHS trying to get them to take it back!)
• Supra manual turbo prop shaft
• Supra manual turbo gearboxes mount.
• BOTH service flaps (for flywheel access)
• Front flywheel cover.
• Both gearbox support brackets
• Lower section of speedo cable.
• Gear shifter
• Gear gaiter and knob
• Rubber seals and metal bracket to hold the seals in place (seals shifter/tranny tunnel gap)
• R154 flywheel (it’s a great opportunity to get it lightened too)
• Clutch pressure plate, clutch, release bearing, and release fork.
• Hydraulic clutch slave cylinder and pipe work.
• Hydraulic clutch master cylinder and pedal assembly
• Manual brake pedal
• Manual ECU
I managed to source all this for ?350
First of all remove the existing gearbox and prop shaft. To do this, first remove the cross bar that sits under the engine block, it’s held on by 2 large bolts and easily comes off! Undo all the bolts around the gearbox bell housing, and the support brackets either side…. you’ll find the top two right next to the bulkhead are impossible to get to undo… so drain the gearbox, remove the prop shaft and gear stick shaft (manual) or gear linkage (auto) then undo the rear gearbox mount, allowing the entire engine and gearbox to swing backwards on its mounts, you’ll now find that a long extension bar with a universal joint on the end will reach those bolts and off comes the gearbox!!! Make sure any earth straps are disconnected before you try and undo the last bolts!!! Don’t forget to disconnect the multiplugs that run from the engine loom to the gearbox. Remove the flywheel.
The R154 gearbox fits straight in in the same manner as the old one, the only difference being the clutch set up.
Bolt the new flywheel onto the crankshaft, without the clutch or pressure plate on it… instead slide these into the bell housing and position the release fork and slave cylinder. Place the clutch disk itself on the shaft that the pressure plate is sitting on (THE RIGHT WAY ROUND!!! But we’ll not go into that one….) and bolt the gearbox onto the engine as normal.
Once its on use the two service flaps as access to bolt the pressure plate onto the flywheel… turning the engine over at the crank and tightening up each nut bit by bit.. It’s a bit fiddly at first but its how it’s done!
Fit the shorter R154 prop, do up the multi plugs and fill her up with oil.
For those of us that don’t have an oil pump for filing up transmissions, you can pour it in thru the hole where the gear shifter goes.
The worst part of this conversion is by far the pedals. I removed the driver’s seat for easier access... but it’s not essential. First swap over the large automatic brake pedal for the smaller manual one, then bolt in the master cylinder/clutch pedal assembly. It’s very simple to do but takes a while... it’s a tight working space down there!!!
Connect the slave and master cylinder using brake line (ask a scrap yard nicely to try and get the original stuff out in one piece!) and fill the clutch reservoir with DOT 3 or higher brake/clutch fluid... the bleed point is on the slave cylinder.
Bolt the shifter on, install your trim and you’re only left with one last job, replacing the ECU. Easily done, the ECU is behind the glove box, and swaps over with 4 screws and 3 multi plugs.
You can now enjoy getting the most out of your supra turbo engine.
Good luck!!!!