Ready to shoot my 273

I agree it is probably a fuel issue, though it sounds more like idling too lean than rich. I base that on my Holley Projection, where I can adjust the O/F with the turn of a knob. If the idle is turned too lean it will idle OK in Park but slow way down and want to die when you shift to drive. My slant six used to do that too until I finally found a good Holley 1920 carb (about my 4th try), then it ran as smooth as a sewing machine. Without a lean/rich indicator (O2 sensor) you are flying blind, so consider getting one eventually.