Thinking about it, if the forum were to be splint into more sections, I think I'd do it by function rather than by era. Perhaps "drive train", "suspension", "body and paint", "interior and upholstery", "road trips", etc. Seems like lots of those apply across both car and truck and across a lot of years.
But, on the whole, I'd just leave the categories as they are. I just use the "view new content" function with the option set to show me anything I haven't read. Usually I don't even notice if the thread is on the truck side or the car side.