20" will fit. If you want to lean more to quieter, dyno max 17747 (2 1/4), a little less quiet, 17748 (2 1/2). It is a bit unfortunate the way the exhaust has to pass over/around the exit tube on the "center" outlet muffler pipe. The Ultra flows avoid that, but they get a bit louder.
My observations indicate that you'll have less (maybe no) drone with headers with these mufflers.
Your 1970 Coronet had 20" bodied mufflers from the factory. They were offset inlet / center outlets. You could run some 18" resonators at the rear, 4" round body or less will fit. The OE resonators were 4" OD. I would go with Dynomax Super Turbos & 2.5" exhaust.
