I looked ALL OVER THE WEB!!!!......and found those numbers on a couple of websites as correct for the 67-whatever, I know that the factory literature says the 67 ups were 4 lower to make it easier to get into, so I think they will work, but may not be perfect.
I went to the NAPA site and put in this search string
1966 >> Chevrolet Truck >> C10 1/2 Ton 2WD - Pickup >> 5.3 L 327 CID V8 and got coil over booster shocks, gas shocks and regular shocks...take a look. I have the 305 v6 790lb motor, so a 327 motor is about as good as it gets.
Napa Response (made by monroe, I think) front RR 94005
Rear RR 9408
There are others listed, but not under GMC. I am going with the NAPA numbers first, I probably will put on a set of the booster shocks on the front, (NS 58241) for 75 bucks per 2 shocks. and gas on the back. Hope this helps, I will repost when I get the front ones in ( I just ordered them) and tell you all how it worked out.
