First a caveat, I skipped probably the last 15 or 20 posts to just get to the end of this thread and add my 2 cents, so if this has already been settled or whatever, please excuse.
using a LoveJoy coupling in (most) EV conversion applications. How do I know? Because I tried it myself, and it ended bad. I'll spare a lot of details, even though I know details matter in such things, but the long and the short of it is that LoveJoys are very good at lower RPMs and torque loads that rise gradually (relatively speaking). In real world conditions of an EV, you've got neither of those, and the pliable center "spider" just eventually gets chewed up. Additionally, the little bit of "play" that you get with a LoveJoy that is seen as one of its advantages in many of its applications turns out to not necessarily be to your advantage, again, in a typical EV conversion. You end up with unwanted unbalance. A solid coupling between your motor and tranny is most desirable. Think about it like this, if a LoveJoy-like connection was what was best between a motor and transmission, then automakers would have been using them (or again, something like them) for a long time now.
These I've found to be great:
Climax Part CC-112-112-KW Mild Steel, Black Oxide Plating Clamping Coupling, 1 1/8 inch X 1 1/8 inch bore, 2 1/8 inch OD, 3 1/4 inch Length, 1/4-28 x 3/4 Set Screw: Industrial Products: Amazon.com: Industrial & Scientific
(Not getting any kickback from Amazon or anything like that, this was just the quickest link I could find)