I need to replace the vacuum unit on my '64 coupe (390 4bbl). Mac's Auto parts has 2 in the catalog - 1st is 58-64 part# 66-31323-1 $24.95 2nd is 60-66 part# 66-37105-1 $29.95. Anybody have any ideas which is correct? Do I just buy the cheaper and save $5? (Both are aftermarket and have a rubber vacuum line connection).