You use the numbers off the q-jet to find the kit. The most likely case is they won't have it in stock and will have to order it.
As far as I know, the kit for a particular q-jet # should be the same regardless of where you get it. Part of that number indicates whether it's a marine carb.
I used to have a Mercruiser 470, and it had the Rochester carb. I just got a carb kit from a 1967 Chevy Impala with the 307 V-8.
Sometimes you can use auto carb kits.
However the gasket that mates the carb to the intake manifold is considerably thicker than the one i removed. Just hope this this not pose a problem