new member in need of mufflers

Hello everyone-Nice to be a new member. My wife and I are proud owners of an original '69 dart swinger 340 L1 beige and black stripe. We've owned it since '76 and did a total resto. in 2002. We drove it as much a possible, but that probably was'nt enough, because the mufflers rusted away and have blown out so bad its too loud to drive. I decided to use Flowmasters when we did the resto, and that was a big mistake. They were way too loud and I never did like the sound. After reading through the threads on "exhaust systems" I've decided to replace the Loudmasters with Dynomax super turbos. I would like to use the 20" case length units, but I read a remark about "they will work, but need some modification". What exactly are the modfications needed?? Thanks Bob & Cindy Richmond