What your smelling is the by products of combustion blowby venting out of the valve cover vents and then being blown towards the cab by the fan. Those fumes have all sorts of ways to get into your cab due to the worn out rubber seals. I doubt they ever sealed perfectly in the 1st place LOL.

I find that I have to occasionally crack open a window after running the engine hard to clear the cab LOL.

I can't count the folks that have remarked when checking out the interior of my 49 say that they remembered that "smell" back when their fathers or others would ride them in their trucks.

My Mrs can tell when I've been on a long cruise by my smell LOL

1949 Chevy 1/2-Ton 3100
1949 GMC 1/2-ton 100
In the Stovebolt Gallery
More pix on Photobucket
Video of the 216 running
Video of a Winter Cruise