I don't know about the GMC, but some of the older engines will run on pretty much anything, so if supply was bad poor quality fuel might be all thats available.
The problem is that unleaded burns hotter, and doesn't lubricate the valves, so burns them out. I'm not convinced this will be a problem for you ? In my classic car, I used to use (before unleaded conversion - hardened valve seats) the castrol valvemaster additive. Mainly because this product is so easy to use. Fill the tank up, note the number of litres, squeeze the bottle and you add fluid to the top chamber of the bottle until you reach the number of litres you just put in, add it to the tank. Dead easy
I hope that helps Jack
Cheers
Rich