I like the TTI stuff and the long case DynoMax Super Turbo muffler (17748). These are like 25" long and I don't know if they will fit under your Challenger.

You could also consider having the local shop bend you some pipes, and you can quiet it down even more. It may, or may not be less expensive. Do a 2 1/2" to the muffler, neck it down to 2 1/4", use the 2 1/4" Super Turbo and 2 1/4" tail pipe.

My experience is that you will have noticeably improved around town throttle response, and you will never notice a power difference.

Even though I own TTI and recommend them to the "B" body crowd, Magnaflow sells a complete kit for 70 up Challengers (stainless steel) with lifetime warranty. Available in 2.5" and 3"

You can call them to discuss technical questions (such as hook up to your hookers) but for purchase, price will be the same as a local dealer. As mentioned, that's their policy.

As you have a stock motor, but with headers, I think we (those who have responded) assume you are interested in maximizing the power while keeping it fairly quiet.

If so, then it seems logical to basically go with a stock style arrangement; but with modifications to take advantage of the headers. While the best source is factory lit (such as shop manual), poking around the web it looks like a 440 e-body came with 2.5" head pipes, mufflers, and resonators and some sort of rectangular tip.

http://www.gardnerexhaust.com/1970-mopare-body-exhaust-system.html

http://www.cuda-challenger.com/cc/index.php?topic=102566.msg1013749#msg1013749

http://www.cuda-challenger.com/cc/index.php?topic=102566.msg1013749#msg1013749

While the Hemi cars were similar they also got an H pipe.
I'd suggest keeping the collector diameter at 3" until it intersects the crossover in the H. Then drop to 2.5" to the first muffler/resonator.

Also, as the most restrictive point in almost any system are the mufflers, this place to look for improving flow at higher rpms. Higher flow can be achieved by larger diamemeter in/out. So if you go with a single muffler each side in standard location like you have now, then definately go with 2.5" in/out. If they are all the way in the back by the tailpipe, the restriction on flow will be minimized because of cooling and some energy will have been reduced in the resonator.

You'll have to do some measuring to see if there is room for an X. I think in this particular situation, an H will serve better (for sound reduction and placement). If placed on the harmonic, which is probably around 20" from the primaries, there is also a possible increase in torque and power.

TTI 2.5 and never look back.

I have Magnaflow on my Cuda, currently 2.5", the mufflers are straight through but its got a very nice rumble, no drone but also quiet which I like, If I stand on it it barks a little, I'll be changing it out to a 3" once the 408 heavy breather goes in.
Ive also installed MF systems on Chev Nova's and like them, built out of thick wall tube, made well, wait till YearOne has a 30% off sale and get them.

Brands and Performance:
Different internal designs provide different results. Some brands offer several designs and most brands offer at least variations within a design. So here's some that have been mentioned.

Flowmaster
Uses internal baffles. The versions that are usually discussed are have V or "delta" shaped baffles.

The gas has room to expand immediately upon entry. These are offered in several series, some (10, 40) are louder than others (50, 70). They also offer several other internally baffled muffler designs. Summary of offerings

Dynomax
In recent years has added several mufflers using different designs. But they started with "turbo" mufflers

"turbos" use internal tubes running through center chamber and reverse the direction of the flow a couple of times. The name "turbo" comes from one of the earliest applications of this design - turbocharged Corvairs. When Vizzard first tested mufflers for flow a few decades ago, several company's models (Not just Dynomax superturbo) did very well.


Ultra Flow mufflers are a straight through perforated pipe with insulation packed around them. In design cncept, they are very similar to

Magnaflow
which also uses a straight through design with insulation.
I suspect the heritage of these designs are the 'Glasspacks. I don't believe the insulation on any of these designs 'blows out'. It seems more likely that it can get moved around a bit and/or compressed a little during initial use.

For the record, Magnaflow also makes a chambered muffler series (XL) and a Glasspack series.

Accurate Ltd
They have had a few different mufflers including some designs based on the stock originals. Here's one from around 2001 for an a-body that someone cut open. Older Stock Based Muffler design (or see attachment) and a newer performance design (click on link for cutaway)
aCM-912
These photos are both from a long thread on muffler flow/restriction on FABO.

https://www.youtube.com/watch?v=_PVXvHkr-Vs They retest this exhaust on a 350 hp small block in episode 17. Virtually no difference in power between 2.5 and 3 in that test. 3 inch was louder of course. Unfortunately, I can't provide a free link to that episode as it is only available on Motor Trend On Demand until next month.