This... I've run engines with the crossover blocked in below-freezing temps and they do just fine, as long as the tune is good you shouldn't even need to spend much time letting it warm up. The last engine I left the crossover open it would frequently boil the gas in the carb even with 1" plastic spacer and the intake manifold would be BURNING hot to the touch. Not good for performance, or really anything at that high of a temp. The only time you might run into an issue is if you're using an Air-Gap intake (which doesn't have a heat crossover anyway) and driving on the freeway in below-freezing temps. But you're in Cali so that's a non-issue.
Hot-air intake, like the dual-snorkel air cleaners from the 70s with the heat riser off the exhaust manifold/header, does more for gas mileage and faster warmups anyway. Modern gasoline evaporates like crazy it doesn't need help like the stuff back in the 60s-70s-80s did.
