Did you try Rare Parts? They show an idler arm available for the 3000 series with the 4000 lbs. front suspension. I looked up the 3500 series and it shows a 5000 lbs. front axle, not sure if it uses the same idler arm. Early 60's GMC's can be confusing, the 2500, 3000, and 3500 series were all available as dual rear wheel cab and chassis models. As far as upgrades are concerned, these trucks have torsion bar IFS, the best 'upgrade' is just to restore what you have. Here is a link to Rare Parts:
https://shop.rareparts.com/search?ye...GMC&model=3000