Single-speed wipers have a 2-wire motor, a simple switch, and a simple connection where the crank arm meets the linkage, while variable-speed wipers have a 4-wire motor, that giant switch, and a fairly complicated assembly of cams, springs, shims, bushings, and spring washers where the crank arm meets the linkage. Grab all of that and you're fine, but you will need the correct variable-speed switch for the '64 (wasn't it?) you're working on -- there are different stem lengths for the different styles of dashboard.

Somewhat off topic, but the 76 D100 I just bought has two speed wipers with no delay. I live near Seattle, so I also deal with lots of rain. I have driven the truck a few times in the rain and I have concluded that the two speed wipers, while better than single speed, are really annoying and I plan on adding a delay function in the future. If you are going to be changing your wiper system, then I strongly suggest you skip the two speed system and just go straight to a modern delay system. I see universal-fit retrofit modules going for $40-$60 on eBay.

much less inadequate

Keep in mind a '76 was built after Federal standards specified minimum acceptable windshield wiper performance. Polara Pat's was not, and the '60s single-speed wipers are too slow to keep up with even moderately heavy rain. The biggest issue with them isn't that they're too fast -- that's easily manageable with the minor nuisance of just flicking the switch every now and then. The problem is that they're too slow.