If you are .030" over on a 340, then the nominal bore is 4.070" and I would use something that has a hole that is at least 4.095 or 4.100" diameter. There can be some mis-registration of the gasket bores on the cylinder bores. And while Cometics have been reused successfully, I personally would not do so on a new build; it adds in another variable or question mark in the new build.
Remind me... are you using stock, open chamber heads?