Never had a Sag 3-speed myself, but as far as I know, in that 1969 era the input shaft would have been a coarse spline as you suggested. Hopefully someone familiar with the M15 can confirm, but a trans shop (or an online vendor's site like Summit or JEGS with an application selection tool) should be able to confirm?
As far as the clutch diameter goes, do you know what size flywheel your engine has, as you'll need to make sure the new clutch matches. In addition, the flywheel may well need to be surfaced, so personally I'd pull the trans to check all the details and condition before ordering new parts. Means more down time, but at least you'll know exactly what you need and won't waste time and money chasing parts.