Hood clearance Question

With the air gap, I'd consider running a fresh air line from the core support to the area under the carb, if I was that worried about it, but any heat soak issues you have are going to occur at slow speed anyway, where a spacer is of limited value in your configuration anyway.

If fuel boil-off is that much of an issue, try buying 100% gasoline, or buy gas from somewhere else.

My first bit of advice, though, would be to build it and see if it's really even a problem, first. Don't worry about bridges you're not going to have to cross.