Blocking the heat crossover, IN the head.

Exhaust crossovers are helpful for the first 2 minutes after a cold start, then it's just robbing power and boiling the gas in your carb. Modern gas vaporizes a lot easier than back when these cars were new. Also the factory iron intakes take wayyy longer to heat up than modern aftermarket aluminum ones. If you really want to get creative and do something that will make a difference, put in some intake gaskets with proper stainless block-off plates then drill and tap the crossover in the intake for pipe fittings and run water through it.

My Holley carbs book by David Vizard talks about this exact subject; he did a test checking the effects of the crossover on power and the water/coolant method actually made MORE power than no crossover at all due to better vaporization as well as keeping the intake plenum floor at a more constant temperature since after the engine has warmed up the intake will ALWAYS get hot from latent engine heat anyways.

FWIW I've driven my Air-Gap equipped 360 in 10* weather in a snowstorm (drift central LOL). It was a bad idea, it took me 10+ minutes just to keep it running but I needed to get somewhere and my driver was in the shop. Crossovers are only necessary if you expect to be driving a lot in sub-freezing temps which you said you aren't anyway. IF it really is a concern get a factory dual-snorkel air cleaner with the heat riser, I run that in the colder months and it makes a noticeable difference vs. my open-element air cleaner.