Nice looking car!

Could your problem be that one or both carbs are running way too rich at idle, leading to fouled plugs? Let it idle until it starts to stumble, shut it off without revving it up first, pull the plugs and take a look. If they are all sooty (could be either the front or rear pair or all four) a likely cause is worn needles or jets. It might also be that the heat problem (float bowl close to manifold) you mentioned is causing it to run too rich at idle, particularly on the rear carb. I've never had a B16 car, so I'm not that familiar with them, but later models usually have some kind of heat deflector under the carbs (which also keeps gas from pouring directly onto the hot manifold if a float should stick or sink)---could this be missing on yours?