I had not switched the bellhousing adaptor. My bad. Next question: the transmission shaft on the donor car side is different -- the splines appear thicker. I need a different clutch frictionplate .... but not sure which one

In MB-speak, the "adaptor" is known as an Intermediate Plate (IP).
In switching from an iron case 4-spd to an aluminum case 5-spd trans the method of centering the transmission axis to the crankshaft axis was also changed by Mother Benz. The iron case type is centered by a circular, peripheral ring between the clutch housing and the IP, while the aluminum case type is centered by a pair of dowels between the clutch housing and the IP. The early IP lacks provision for dowels, hence the IP from the donor must be installed, and centered to the crankshaft.

Re: Input Shaft
Measure the OD of the splines, and count the number of splines. Also measure the OD of the friction disc of the original installation, if that is the source of the flywheel and pressure plate to be used.