Vacuum advance cans come in a staggering array of different characteristics. Some more of less vacuum, others more or less degrees of timing. I think this is what he meant to match the engine and the work load it will be doing. My little 327 with a big cam and heavy truck, I had to tune the ignition so that the vacuum would retard alot under heavy load (lo vac). Once you get it it is a beautiful thing.
Just go to the local auto parts store and they can fix him up. Enjoy!