1989 driveshaft to newer year transmission

[dccse3k]

So i switched transmission from an auto 1989 transmission to a newer auto transmission (im not sure of the year probably older than 92 and newer than 90) and it is very difficult for the old driveshaft to slide into the new one. It is a lil rusted but the thing that concerns me is that on the driveshaft spindle gear thing has 2 flat spots and the spindle gear on the transmission has none. the splines seem to match other than the flat spots am i sol?

[biggie]

Seems like the sol is true.

IIRC '91+ AT wouldn't work on SOHC engine.

I'm sure AT suff is plentyful around you, I've got all my AT stuff for free and no one wants it.