Exhaust is going on this spring, finally. 70’ Coronet, 440 using Doug’s long tube headers, I’m thinking 2 1/2” off the collectors,then x-pipe and on to the mufflers. Looks like b-body mufflers are center in offset out and reduce to 2 1/4”, I’m planning to use the chrome tips. How big a muffler body can I use, I see them from 14” up to 20” or so. Is bigger necessarily better? We’re planning long (weeks) road trips in the car so not interested in any “drone” at typical highway speeds but would like that nice muscle rumble at idle and slower driving. I realize sound is a very subjective thing but any of your experiences with your exhaust would be much appreciated.

The longer the muffler case, typically the quieter it is. Same goes for smaller OD tailpipes.
Going too long though complicates the tailpipe bend after the muffler as far as clearing the shocks & rear axle.
I'm usually 18" max length on mufflers, and for something you don't want to be too loud, look at:
Dynomax Super Turbo
Magnaflow equivalent
Walker "Hemi" muffler
I liked the tone of the Dynomax on a fairly healthy 383 RR, not tinny, not too deep, was throaty & crisp.
Ran the same on a healthy Hemi RR: headers, H-pipe, full 2.5", modified (2.5 inlet) tips, perfect.
The tips seem to sound better than non-tips, my guess is because of their chamber.
X-pipe will usually have the effect of smoothing the pulses and giving a higher pitch to the sound. H will leave most of the pulse in there.
I'd be tempted to run 2.5" tailpipe on a 440, and tips are available nowadays for 2.25 & 2.5 entry.
That said, sound is kind of subjective: on a wagon I did, I tried 3 types of mufflers before I was happy; my exhaust guy still chuckles about that one, but the architecture of a wagon (they're like a drum) complicates that.

Which type and what size motor?
If it is a 440 motor and you want the best performance and are willing to mount the mufflers behind the rear end then I would use 3.0 inch kits form Summit and one of several different mufflers brand and type depending on the motors power, no flow stoppers muffs though

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.

The bold part,you bring up a good point however the problem is not as big of a problem on the bigger body mufflers.

For the kind of sound you're looking for I would go with one of the 20" dynomax super turbos.

I think you might enjoy an H pipe a little more than an X style, although I prefer the sound of the X style, I always thought it sounds less old school than the H.

Coming off the collector, Flowmaster sells a reducer cone that drops in size a little more gradually than the norm. It might be a good idea to try those.

I replaced a set of 14 inch case Thrush Hush turbos, which neck down to 2.5 or 2.25 inside, with 20 inch case Ultra Flos.

3 inch pipes, dumped at the axles, the longer Ultra Flos are quiter at idle, quieter at cruise, way less drone. Run the longest case you can.

Thanks for the replys, very helpful! Been talking with a local shop, gonna run 3” off the collectors, x pipe and into the mufflers. Sounds like a 20” body should give me the sound/tone I’m after. I’ve always like the sound of Dynomax so gonna use those, offset in center out. 2 1/2” tailpipe out of the muffler to a set of stock looking bright tips.