Factory tailpipes were a two piece welded unit. 1969 units front half exiting from the mufflers were 2-1/4” O..D. The rear half was 2” O.D. which was slid into the front 2-1/4” section and welded. The rear where the exhaust extension was attached was opened to 1-7/8” I.D. for the chrome extension to slide into. By the time the exhaust exited the hole in the exhaust extension it was only 1-5/8” I.D. Not much flow. Big block tailpipes and 340 tailpipes were the same units. In 1968 the tailpipes were 2” O.D. exiting the mufflers and sized down to 1-7/8” at the rear. 1967 Big block tailpipes were also 2” but were only turndown style without extensions. Hope this helps.
https://www.accurateltd.com/One-Pair-of-Tail-Pipes-for-tips--225-or-250-A-Body_p_202.html
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