340 misfire

A compression test wont help with vacuum leak but will tell you if you have bad valve, valve hanging open or weak cylinder from something else. To check for vacuum leak let car warm up, let it idle and spray carb cleaner or water around the carb base and intake where it seals to heads and block, if your idle speed changes noticeably you have a leak some where. Compression test take out all 8 spark plugs and test each cylinder with throttle wide open.