3 different years, same transmission, which one should I use?

Use the 76 tranny - it should be a part throttle kickdown unit which is preferred over the other two and will be a 904 unless it was messed with. The 66 tranny will have a small torque converter pilot hub and I think a smaller main shaft diameter, this will make hooking it to a later 318 troublesome. The 68 would be okay - but not have the part throttle kickdown. Ask any of the trans builders like A&A, they recommend the later part throttle unit. Save the 66 trans to match your 273 and set them together in the corner if you like. If your car is a 66 or older - the 727 is a tight fit in the dog house as well, for this reason the 904 makes more sense. In addition the driveshaft needs no mods to fit, and your trans linkage is a snap to install since it was a 904 orignally, just use the 66 linkage on the 904 you choose.

Enjoy !