The MS4 transmission can be replaced by the ZF 45a but it's not an exact replacement. Both transmissions have an 8 degree down angle, but the ZF has slightly more drop center, about 3/4 ", and it is also about 3/4" shorter than the MS4. Unfortunately I am not at home right now and thus can't give you the exact dimensions. In any event, this means that the engines will have to be raised and moved rearward these amounts inorder to match the gearbox output flange to the existing prop shaft flange position. Alternatively, you might raise the engine and slide the shaft forward if you have the extra length, or fit a drive saver if there isn't one there already.
The MS4 transmission was a mechanical gear
with a cone clutch
, and the forward and reverse reduction ratios were identical to each other, but the ZF is a hydraulic actuated gear, and as I recall
there is a slight difference between the forward and reverse ratio. This shouldn't give you any problem and I doubt you will have to do anything with the props.
Another thing that is different is that the ZF gear uses an external oil
cooler, as opposed to the internal cooler on the MS4, so you will have to make some changes to the raw water plumbing
lines. (Don't forget to buy a cooler and hoses along with the transmission)
Lastly, this would be a good time to check and probably replace the torsional spring plates on the flywheels, especially if they are the originals units. These things don't last forever.