I think the best choice is to narrow and section the one from the 427 Baldwin Motion kit. I think this will be a better starting point.
I also thought there was a convertible up top in one of the 289 kits but I didn't find one in the latest release.
Put me down for the correct hardtop for one of these.
By the way the color for the Castlow & Larson cars is House of Kolors F21 Fuschia Dry Flakes over black.
When I talked to Harold Bradford (Historic Racing Miniatures) about clear resin. He said all of the "clear" he has tried eventually gets a yellow "tint" to it. Maybe Robert (GM Future Liner) had different sources since he works in the industry now.