Yeah, it wasn't until the '77 downsizing that all the B-bodies across the CPOB brands standardized on a single wheelbase. Prior to that, Chevy had one wheelbase, Pontiac had two (both B-bodies), Olds had two (B- and C- bodies), Buick had two (B- and C- bodies)...and even more variation once you get back as far as the early 50s and before...
As far as the 60s Pontiacs went, Catalina and Ventura were on the shorter wheelbase, as were the Grand Prix and 2+2. Star Chief (renamed Executive) and Bonneville were on the longer wheelbase.