Hi
I installed a brand new GM T56 Viper Tranny with the adaptor plate, so it fits the original manual clutch bellhousing.
6th gear is a 0,5 : 1 overdrive and I finally changed rear end from 3.70 to 4.11 to have better highway RPM.
The L71 likes to run 2000 RPM, so that's now perfect at 130 KMH.
I bought the tranny with adaptor plate and mechanical speedo output from D&D for 2200 USD, had to buy a new clutch because of 68 had different spline clutch ( but who would install a new tranny to a old clutch anyway ? ) and a 1310 Mopar auto slipjoke to clear the tunnel. Cut out the manual welded in crossmember and installed a auto crossmember slightly modified to adopt the T56. Shortened the existing drive shaft and all together was far less then 3000 USD.
The tunnel had to be cut a bit, but with the fiberglass tunnel, that's a piece of cake to do. Modified the original M21 shifter handle to fit the T56 and all looks original from the inside. Absolutely nothing visible unless you lay on the floor and check from underneath.
Would I do it again ? Yes, but mainly because of the cool factor. A TKO 5 Speed is more then enough. But unfortunately, KEISLER's TKO is more expensive than the strong version GM T56 Viper tranny from D&D.
If you check up the US Forum or German Forum and use the surge , you find plenty of pics I took during the mod.
By the way, of course this was a 4 speed M21 to 6 speed T56 conversion.
Rgds. Günther