Jim -
The basket drive part number for the whole assembly is/was 383927 for your exact configuration. If you're going to buy a whole drive, you want the 18.5 inch spin tube (its not 18.25 as I thought earlier), the green (for large capacity) brake springs, and the narrow pulley. This was not a high volume usage part in this configuration, as I can recall only the 1974 Lady K, the 1974 Kenmore 70, and the 1975 Kenmore 70 as having used it, in addition to your whirlpool and possibly a few others. Nothing in the Kenmore line used it that I know of in 1976 or 1977, and the spin tube length changed to 13.something inches in 1978 and thereafter.
You can follow John's suggestion and disassemble your working basket drive if you ever need parts, and you should be able to buy some components separately such as the clutch pads and drum, etc. in addition to the spin tube. If you've taken the rest of your machine apart this far, the further disassembly of the drive shouldn't be a big deal for you. You'll need clamps or a vice to re-seat the set screw. It was this set screw that prematurely failed (it un-screwed and ruined the spin tube) in our 1974 Kenmore, which started this fascination of belt-drive repairs for me.
As to a replacement base plate, the 4.5 year availability of your configuration (from later 1973 thru 1977) is going to make finding a good one 36+ years later kinda tough. You take a risk of finding the same conditions in a donor of the same age as yours. I think I would try John's suggestion and weld-on a stainless sleeve using your existing post --- from what John has said it worked flawlessly on his now very rare 1968 KM. To me it seems like the perfect solution if you want to use this washer for a LONG time as stainless is better than original in this case.
Talk to you soon Jim.
Gordon