For what it's worth, when GM sold their Heavy Duty truck line in about 1986 to Volvo, the parts responsibility passed to Volvo. The new company became Volvo GMC and later evolved to Volvo White. I think it's Volvo now and I would think this is where you should look for parts info.
I was a Chevrolet GMC Heavy Duty dealer then and it was a major blow to us because we weren't able to get a Volvo GMC Franchise.
Hope that helps.
