So my primary belt is too tight and the transmission adjusted all the way forward, so I decided that a 39 tooth front pulley would be the way to get everything to fit.
For what it's worth I was running a 62-40 SK3 kit with the BDL-30853 BE "Bullseye" Belt. BDL number convention breaks that out as 62 tooth clutch basket, 40 tooth engine sprocket, S for splined engine sprocket, K for kick only(no e starter ring.), and I have no idea what the 3 means. BDL lists this as a kit for 1965-1978. It's too tight on my rigid frame of unknown origin.
Swapping the engine sprocket to 39 tooth will give me the same set-up as a 62-39 SK2 kit which BLD lists as fitting a 1955-1964. As it turns out this is the kit I should have started with.
First let's look the actual diametrical differences between the 39 and 40 tooth pulley. The 39 is smaller of course.
With just a very basic reference measurement the 40 tooth is 3.985 inches across.
Using the same methodology the 39 tooth is 3.890 inches. So about 0.100 inch difference; which doesn't seem like a lot.
Here's the cold belt tension with the 40 tooth sprocket.
Here's the cold belt tension with the 39 tooth sprocket with everything else the same. That's actually a bit on the loose side.
Look at how much slack there is. I'm considering running a standard belt instead of a bulls-eye because it actually fit just about right.
Time to tear down the clutch and swap the transmission 23 tooth sprocket for a 25 to get my gearing back in order. Ugh, more unplanned stuff. The lower left stud has decent enough thread to hold things together, but the stud has a bunch of lateral play and is leaking. I knew this already and tried to fix it with plumber's tape, but yeah that didn't work. Now I need to fix that. Stay tuned.