I installed an aftermarket (Dana) CC on our 83 a long time ago and it's still working OK, and is a great thing to have on longer highway stretches. Installation can be a DIY job if you have some moderate mechanical and electrical know-how. The biggest challenge was hooking it up to the Volvo throttle drum. The stock cable from the gas pedal will go slack when the CC's cable/chain pulls the throttle open, and you have to make sure that it doesn't then get hung up on the rim of the drum or something and stick open. The factory CC's have a different (2-pc throttle drum) setup so that the gas pedal cable and CC cable operate independently. Unless you can get one of these, you are going to have to jury-rig something to keep the cable in its track (which is what I did). Whatever - make sure it's a safe setup.

I've heard good and bad things about the durability of the JC Whitney CC's. You get what you pay for - maybe their pricier units are OK. An alternative may be an all-electric (no vacuum) unit. Take a look at www.speedometershop.com and go to their cruise control section. This is an LA-area business, and I have no experience or connection with them - just found them while surfing, but they have a building, they have an address, and they've been in the business for quite a while.