I've seen it make a big difference in coolant temp, i've seen it make not much. Depends on how much advance you are starting with and how much vacuum adds. Stock slow curve 5 initial 30 total type setups it can be a huge improvement.

As Andy noted it WILL make a big difference in exhaust temp, which will reduce your under hood temps. That sounds like the real issue as far as your fuel boiling.

Are those headers ceramic coated, or just silver painted? Header wrap near the fuel line could help, if they are just painted you might want to consider wrapping the whole headers.

Are you still running a drop base? A flat base will also make a bigger difference than one might think keeping the carb cool as well.