Which A-bodies do big blocks fit best in?

NONE. They're all a *****.
Take ya at least a day to remove the engine, but a small block comes right out. Headers are a nitemare, no matter what brand. You will be heating and pinging them to fit for days.

Take it from a guy who has raced a 440 Dart for years...Getcha a hot small block.
You'll be one very happy Camper.

View attachment 1716085213

View attachment 1716085214
I have to disagree. I did a 440 in a 74. kept the power steering. Shumacher headers and make my own power steering pump bracket. The cast aluminum valve covers hit the bigger blower motor of th eAC car. I modified a fan motor with a universal squirrel cage. EASY.