65 and older 904 trans is a cable shifted trans and has a small input shaft/torque converter pilot, flanged output shaft.
66-67 904 trans has a "linkage" shifter, and a slip yoke output shaft. Still has the small spline input shaft and converter pilot.
68 up 904 went to the large spline input shaft and torque converter pilot.
Also I think the speedo cable connection changed somewhere around 65, 66, and 67
not sure of the year, but sometime in the '70's the 904 had a "lockup" converter, which has a different input shaft spline, and converter.
You can put a 68 and newer slant six engine, in front of any slant six 904. If using the 67 or older 904, you need an adapter bushing in the end of the crank.
If you want to use a combination of early (67 down) engine, and late (68 up) trans you need a special built torque converter (not many converter builders will do this any more), or use a later crankshaft, or have the hole in the end of the crank made larger.
Keep in mind if using a late 904 (68 up), and engine in a 65 down car there will also be linkage issues, and a different driveshaft.
Not easily. engine has the small converter pocket, converter has the large pilot.
