I installed Audiovox kit on my '84 D100, 225, 833od. It works great. This kit appears to be out of production.
To ease the instalation of the bracket to hold the servo throttle cable, I grabbed the OEM double cable clamp from a junkyard slant six with factory cruise. The clamp holds both the original foot pedal and the new servo cables. I think the S6 and V8 used the same double bracket.
The biggest hassle was mounting the magnets on the drive shaft and the bracket for the pick-up. There once was a GM Signal Generator that was used on early '80s Buicks that attached to the trans where the speedo cable went and the speedo cable then attached to the GM SG. This SG would make for a clean and simple installation but it is no longer in production.

EDIT: Here's a Audiovox kit like I used selling on Ebay.
EDIT: Here's the Echlin VSS232 replacement Buick SG that is discontinued. Find one and it will make the install easier.