Does anyone know for sure if the rear quarter glass from a hardtop will fit a convertible?

Stan,

The quarter glass is different between hardtops and convertibles. If you need new 1/4 glass for your convertible, I'd suggest calling either Pilkington Classics or Auto City Classics. Last time I checked with Pilkington, they had OEM LOF glass for 70-72 convertibles for either $125 or $1250 each and they can date code it for $50 per piece.