Jed,
To avoid heat soaking a carburetor, I recommend blocking the exhaust ports into the intake manifold, or get one without those openings and go to a manual or electric choke. I found that there is plenty of heat to the intake, like 115*F after 20 min. warmup on a cast iron, 4 barrel intake, on a 273. What kind of chokes have you used so far?
I have found that old carburetors often have sloppy shafts that leak air and fuel and are not worth repairing. Maybe you can find a good new one, unless you've positively decided to go to a 4 barrel. In that case, what 4 barrel intake do you intend to get?
I guess you are familiar with problems with using old gas and the common, poor quality California gas available to us.
