Traditional to me would be cars built post WWII 'till mid/late 60's, like you see in HotRod Deluxe magazine. It had a lot to do with what was available, (cheap!) and depended on the builders skill level, circle of friends and tools. Like Don said, flatheads were tossed, new OHV V8s were shoehorned in, SB Chevys really didn't become widely available and used till the late 50's. A guy with a stick welder, Oxy Acetylene torches and an actual garage solely dedicated to building hot rods was like a god.
I think the muscle car era of cars available from the dealer with big blocks, 4 speeds and so on almost killed the traditional hot rod. Movies like American Grafitti, California Kid, etc., and dedicated magazine guys like Tom Medley and Tom Mcmullen turning out Rod & Custom and Street Rodder magazines basically brought about the resurgence of the hot rod in the 70's and gave people the nostalgic look back at the 'good old days' of traditional build styles. That style of course varies depending on what part of the continent you're talking about, big differences in east versus west coast style.
I like just about anything with spark plugs and wheels...