Why use a car blower motor in the first place? Many cars now have powered seat adjustment motors. Top line old Ford Grenadas had them (Thanks to Brian_c for this). The motors are powerful, fit the original questioners diameter requirements and BEST OF ALL come with a lovely spring coupling that obviates the use of "universals". I use these motors in all my "heavy" ships and am really chuffed with them. Loads of power, behave beautifully with a decent speed controller (I use RC Lines units), low power drain, don't get hot, and have a well designed mounting plate attached which makes fitting into the hull pretty easy. They are polarity reversible as well. As always, there is one teeny weeny thing you will have to do to fit the spring coupling. The spring coupling has 1/8" square ends. My method of fitting is to use a 1/2" length of ally. rod one end drilled to fit the shaft diameter and the other drilled and filed square to fit the spring. The round end fitted with a "grub" screw as usual.
Slightly different but still on prop shafts. Why go to the trouble of soldering grease nipples etc. into the shaft outer casing?
Drill a hole in the outer casing (as one does)and then do one of 2 things. 1). Remove one of the shaft bearings ( they are only a push fit) and slide a short length of close fitting brass tube on to the shaft. The tube can slide over the hole.
2) Drill the hole and bung a "Terry Clip" over it. Both work well but the clip is simple and easy. Of course, your grease gun will have to be fitted with a suitable brass tube "probe". Just trying to be helpful Bryan Young.