Production totals. No matter what, any combo of options is a WAG.
The only production totals known is the number of engines built and the number of hardtop vs. convertibles. When you said, "Most L6 cars weren't convertibles. So that leaves 350 and 350HO as the volume convertibles", this is an opinion and not supported by any released data I know of.
69 Firebird 350HO Palladium silver convertible 4-speed and 69 Firebird Champagne with a green convertible top 350HO automatic.