I havn't seen any bellhousings for a T-5 to bolt up to a 460, but even if you found one the T-5 would get trashed pretty quick. T-5's can handle aroun 330 lbs/ft of torque, a stock 1970 460 puts out around 360 hp-480 lbs/ft torque. If you still want a manual, check out 4 speed toploaders. I looked into them a while ago, they can handle a lot more than a T-5, but they don't have an overdrive gear (although neither does a C-6), so if you don't care about overdrive, I'd go with one of those or the C-6 auto. I heard it's a little more complicated to fit up the Toploader as far as the headers clearing the clutch linkage, etc but it can be done. The toploaders go for around $900-1000 w/out the bell housing. Check out
www.4speedtoploaders.com , they have toploaders + parts for swapping them into mustangs w/bigblocks.
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86 Mustang GT w/pretty stock 93 5.0 (cam,pulleys,injectors, throttlebody,h-pipe)
Motor to be swapped for a 1970 429 +.030, ported heads, solid roller cam, victor intake, 10.2:1 comp, C-6 trans, 3:73 rearend W/moser axles