Naturally, with everything going so well, it was the perfect time to realize I made a significant mistake in this whole process. I ordered the wrong crank for this motor, not paying proper attention to the fact that some Sprints' cranks had the later PX flywheel cone. Some did not. Mine is the Did Not kind.
Lucky for me, I can spend my way out of the problem, but it's going to cost me $200 (with shipping), a week of time and a lot of self-flagellation.
So assuming no one tells me not to, I'm going to order the Mazzi long stroke crank shaft that has a P series cone for the SIP Ignition, but fits the old seal.
It was a dumb mistake on my part to forget to verify rather than blithely trusting SIP's fit list for the part rather than using my eyes & brain. I don't know that I'll ever build up a P series motor with a rotary on it, or if I do, it'll be a P200, so I'd be willing to let it go relatively cheap to someone so I don't have to look at my mistake on the parts shelf every time I'm in the garage.
In the meantime, assembly of other components continues. Cruciform swapped, axle and drive shaft bearings installed. Crank seals, xmas tree and gear stack will go back in today.
Assuming no more parts ordering failures on my part, short of crank installation and any adjustments that the new crank requires (I'll have to measure stroke again, but I'm not going to bother until I get the new crank), this thing should be ready to seal up.
Original vs. new-but-incorrect, crank shaft.