I have no hope of getting one of these trans, so I'm seriously looking into this:
https://shiftsst.com/articles/cat/mopar/post/challenger-barracuda-1970-1974-perfectfit-kit/Tremec T56 Magnum Kit Components
Tremec T56 Magnum 6 Speed Double Overdrive
PerfectFit Shifter position fits inside the factory console
700 torque capacity wide or close ratio.
Friction Lined Synchronizer Blocker Rings come standard for easy high RPM shifts
Full 12 month warranty with unlimited technical support, 24 month installation window
Optional Add the Front Shift Kit for bench seat clearance
SFI Rated Steel Bellhousing
Hydraulic Clutch Actuator Kit
6-Speed Pattern Shift Handle Insert
Sheetmetal to widen tunnel
Speedometer Connection
Mechanical and electronic compatible
Speedo Gear
Reverse Light Connector
Pilot Bearing
Super heavy duty needle roller type with lube seal for long life
Crossmember - No need to replace torsion bar suspension. Stronger than the original equipment crossmember.
Isolator Mount
Rubber for vibration isolation
Installation Hardware
Slip Yoke and Driveshaft
Detailed Installation Instructions - including free lifetime tech support
12 Month Product Warranty
Includes 24 month activation delay so you can finish your project.