If the Magnaflow tech said mufflers & resonators are the same thing, that could be true sometimes, but also not true a bunch of the time. If you were going to run a muffler & a resonator in the same system or bank, the muffler would not need to be straight-through, but resonators are always straight-through. Resonators should not add more restriction to a system. You could use Ultra Flows as resonators, glasspacks, Corsa, etc. - as long as they are straight-through. Beware though, many glasspacks downsize internally & that will build backpressure. On your Charger exhaust, you'll have room for 18" long body & 4" diameter - maybe a little larger diameter. On the noise control package from the factory on these cars, the resonators were 4" round & the bazooka tips connected directly to them via sheetmetal screws. I did a very similar setup on my '73 Rallye 340 car using Walker 4" round mufflers - see photo.