Hi, capn_r -
The transaxle input shaft is splined, yes. But the lovejoy coupler was modified to fit - it didn't come this way.
We scavenged the splined clutch hub from the Geo's clutch disc, and then bored out the lovejoy and had the hub welded into it (and then ground it smooth & spray painted it, which is why it looks OEM).
We got this idea from Dr Larry @ EVconvert.com -
http://www.evconvert.com/article/larrys-ev