Looking for the title of a book likely published around 1980-1995 or so where the main character drives an old (1950s or 1960) pickup truck down a time travel "road" that others travel as well. He is chased throughout the story by a dragon of some sort I think. Cover showed the back of the old truck and maybe guns or a dragon tail. Main characters were an old or older man I think, traveling with a younger man or boy. Not sure about that. The opening sequence has the older character stopped by some sort of time travel road policing agents as he is trying to take guns back to Sparta/Troy? to help them win a battle.
Probably Roadmarks by Roger Zelazny.
From the Wikipedia summary:
The narrator and protagonist, Red Dorakeen, has vague memories of a place or time that is no longer accessible from the Road. He runs guns to the Greeks at Marathon, trying to recreate history as he remembers it in an attempt to open a new exit from the Road to his half-remembered place.
Roadmarks by Roger Zelazny. The cover has the blue Dodge pickup truck and the dragon exactly as you describe.
As noted in the Wikipedia article, early in the novel the protagonist is trying to run guns to the Greek soldiers at the battle of Marathon; that matches very closely what you remember as well. (He's trying to push history back into a configuration he remembers, which includes the Greeks winning at Marathon.)
The younger man may be Red's son Randy (C Twenty Cleveland) who goes looking for his father with the help of the AI called "Flowers of Evil."