I'm not one one for making cars all original either, but I certainly wouldn't ever sell a numbers matching drivetrain to a car like that.
I'd keep that block, no matter how bad it is and transmission with the car.
After all it's either a 440 4-speed or 426 HEMI 4-speed to have that transmission, right?

1969 Dodge Super Bee A12 (440 Six Pack, 4-speed, Dana 60 4.10)

1972 Plymouth Road Runner (400, 4-speed, 8.75" 3.23)

1974 Plymouth Duster 360 (360, 4-speed, 8.75" 3.23)