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.
Anything other than high level numbers are a guess. Unless PHS and crew have gone through every record and categorized them. It would take forever but would be much closer to definitive than the X% were this and X% were that so maybe X number of cars were optioned this way methodology that is used.
My WAG is that you will see a lot of clustering, similar options chosen on many cars.
1968 400 Coupe, verdoro green, black vinyl top, auto 1969 GTO Judge coupe, Carousel Red, manual 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver), Fire Red, white top, auto 1972 Buick GS Stage 1, Royal Blue, black vinyl top, auto
