Power brake vacuum source question

[Metalaroundstone]

I recently bought the stock power brake linkage for my 66 C10 and am getting ready to install it along with a new booster. My manifold vacuum fitting (behind the carb has two fittings, one of which goes to the transmission, and the other one is unused. Lucky! Unfortunately, both the fittings are 5/16", whereas the vacuum booster fitting is 3/8". Do I need to get a new fitting somewhere? Or is there some other way to deal with this difference? Does many body know how vacuum line was handled on an all original 327 with power brakes?