David, glad you're back on it!! Looks GREAT! I built the 1959 Impala Convertible and don't recall {several years ago} any fit problems. But I've slept since then.
Dennis
In my opinion, I would have made a step cut in the quarter panel and moved the rear wheelwells back the amount that you extended the quarter! it almost looks like a altered wheelbase car. Excellent work so far!! Just my opinion.
Dennis