The surest-fire way (ok, so that isn't a real expression) to have random life crap overwhelm my shop time is to post some questions here and get good help that deserves responses. Oh well, I'm sure you all experience it too.
Chris - yes, it fits. Sorta. Smaller diameter than stock but a little longer with the attached fan on the back, though I can shorten that end of the shaft (it is oddly long). That way I'll just make a louvered extension on the motor pan that sticks out around an inch - won't interfere with anything.

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Bill! I'm always glad to get your input!
I'll PM you about the motors you still have. I can't afford a PowerPro at this time, someday!
I think I'm on the right path since it sounds like I'm doing much of what you say
I've assumed I would mount it to the upper casting, hanging upside down, though I may mount it in the motor pan - depends on what is easiest and OK. Sounds like this is similar to what you did, if I understand correctly. Do you have any photos?
Turned out the pulley isn't appropriate as is.
It is spot welded to the nut so that it doesn't turn, but there is nothing to keep the nut (and pulley) from spinning off when run in reverse (which is the normal way for SS, as this motor runs the other way stock). The shaft is .67" or 17mm, luckily standard for alternator shafts, though there is no keyway etc.

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I just purchased a replacement:

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. I'll be able to drill and tap it for a set screw and grind a flat on the shaft. I can also grind a slot in the shaft for a key though I hope to not have to bother. I also got a stop collar, thinking I would just use a key and capture it between the stop collar and the nut, though I'd like a simple set screw instead. I also checked the minimum pulley diameter for A size link belts and this is it: 3.1something. The other was too small, and 3/8 instead of 1/2 wide belt. I figure I'll start with HF purchased link belt to get it all set up, then switch when I know the size so I can conveniently use reverse.
I love the idea of using the control knob with the electronic speed control's pot. I plan on adding a tach, something that it doesn't have.
It also needs the reverse switch. I've tested it, just needs a switch. I saw a post somewhere else (don't know where, argh!) in which they had a safety toggle switch that didn't allow going from forward to reverse or vice-versa in one movement. You had to put it in the central OFF position then move it again. So far I can't find one, probably because I don't know what terminology to use in searching for switching. DPDT I know but only because I looked it up last week.
I have no sense of whether this will provide enough torque. Hope so, but I'll find another use for it for sure if it doesn't. I got it off craigslist for $120. I had looked at salvaging a treadmill motor myself, only that was just low enough a price that I thought it worth it: means not having yet another big collection of parts of something in the house to drive SWMNBPOTOOILMT* to distraction. He's been messing around with them for a while, it seems, and did a great job overall - though I'm changing a couple things. He shipped it super fast and has answered a number of questions I had along the way.
I'll keep you posted, though it will be a bit. I'm doing a full shop re-planning and rearrangement, getting ready to make a hoist (ordered block and tackle for that, too) to move the SPTs around, just took apart my SS jigsaw (Yay! The puffer's leather cup gasket seems to be in great shape!)... so I've got too many fires burning right now. All part of an attempt to make it so I can just walk in and work and not have to move things around a lot. This will let me keep two SPTs mounted on the second Mark V at all times. If I'm really lucky it will run the jointer and planer, though from Bill's comment I suspect that is asking too much.
*she who must not be pissed off too often or I lose my toys. OK, so that doesn't exist as an expression either.