Hi Gordon
Great progress, you will definitely want to figure out a brake booster for your truck. Though these trucks do have good breaks, particularly when compared to other trucks of the day. Good working booster and keeping the brakes in adjustment are critical to being able to stop them.
The C60S or C60L with larger drums, as empty trucks are not bad without the booster, but with a load or body you really need the booster. Take it that you are missing the rather complicated linkage assembly for the booster as well.
I assume you are looking at some of the hot rod brake booster conversion, do keep us posted on what you work out for a booster.
Drove my C60S with the radio box up and down a auto road on a mountain near here yesterday, no problems going up 2nd-3rd gear going up but kept it in second coming down just to minimize the reliance on the brakes of stopping 10,000LBS of truck.
Cheers Phil