I found a 3615S, which is for a 64 383/413. Its specs ~625 cfm vs 500 cfm.
I was able to find a 3854S but unable to obtain yet. But the 3615 and 3854 use the same bolt pattern, just slightly larger internals. I found a couple part numbers.. 2468738/170ba-817s
The secondary butterflies may open with that carb on a 273 intake but the secondaries probably won't open fully. The 273 carb has small butterflies and corresponding holes in the intake. The big block carbs are larger.
