there's quite alot to consider before undertaking this conversion,i am currently in the planning/prep stage with this box on my xr4x4 twin turbo,if you weld the bellhousing then it has to be spot on,and you have to take into consideration that the bell housing will warp slightly when being welded,so allow a bit of room for manovure.
i got my r33gtr vspec box input shaft ground down to 14.9mm so i could keep the standard v6 spigot bearing,the v6 bellhousing was machined together with the r33 box and you have to get the input shaft protrusion right,adaptor plates could be made but it will just make the gearbox longer than it already is but a big plus would be you won't need to strip the box down,you would just need to bring the spigot bearing out with an adaptor,if you move the box back then the clutch arm travel will also be moved,loads of thing to watch out for on these.
I know welding is the only way i could do it,no off the shelf parts for the old v6,a proper cast would be a bit stronger but would not be worth it unless i could sell a few hundred off them,not worth the hassle either,i have gone for a pull type twin plate and modified a 2.8 flywheel as no matter what i did the 2.9 flywheel was too thick and the push type slave cylinder would not sit right with front diff and exhaust,i hope the clutch is good enough as i've only got 280-290bhp but nearing 400ftlbs of torque,hence why i'm doing this conversion,broken a few mt75's and can't really afford 5k upwards for a decent box.
