For a 2nd gen Plymouth A-body:
51 " would be about right for 3 speed manual, or a 904 with a small dif.
from the '68 Plymouth FSM
Manual Transmission 3-speed; 170, 225, 273, 318 C.I. Engines 7 1/4" Axle
51.43"
Automatic Transmission;
170, 225, 273, 318 C.I. Engine 7 1/4" Axle
51.43"
Driveshaft length given from centerline of front yoke bearing bores to
the centerline of the rear bearing bores.