As suggested by MikeE, pressure/power wash the crud away. It was 2-3 inches thick under my COE's entire body/frame (due to a badly leaking seal at the harmonic balancer).
I had to rent a 210 degree-F pressure washer - it removed all the crud, oil, c grease, etc. My regular pressure washer was not strong/fast/thorough enough.
My 1954 trucks are clean, with no gas leaks (a few small leaks from the drive-train), and they have no foul odors inside or out. I'll be placing heavy duty disposable diapers under those small leaks (in my new/clean garage).
