The shift foot should be point forward on the upper half and should be held in position mated to the shift shaft on the bell housing
The prop isn't staying lock in rev so the shift shaft is moving. Could put it in position and then use a bungee cord on the prop to keep the prop locked in place
The splines on the lower shift shaft will mate with the upper shift foot, and it might hang on the 2 pieces if they don't align correctly. It can be done but sometimes it just not worth the trouble for me to save the gasket on the upper.
The easiest way I've found to do it is pull the upper, mate the lower with upper and than install drive