I'm putting this post in for anyone that might be considering a 3 speaker dash. This one happens to be for a 70 Challenger.
Although they came from different sources, the pair of 3.5" side speakers and the center 4x10" speaker all were made by Custom Autosound.
The small speakers: http://www.classiccarstereos.com/category/1970-1981-Camaro-Speakers-Mono.html
Look at the list for speakers that end with "1013" - there are two choices. The speakers are easily removed from the metal brackets shown in the pictures.
The larger speaker: http://www.cjponyparts.com/catalogsearch/result/?q=4x10+speaker
Yes, I really did buy parts listed for a Ferd and a Chebby for my Mopar. I will go do penalty pushups immediately after finishing this post.
There is also an option to put in a pair of small speakers in the 4x10" space. There is a person on "forabodiesonly.com" (" '74 Sport ") that sells these adapter plates for $26 including shipping. This link should work to get to one of his posts:http://www.forabodiesonly.com/mopar/showthread.php?t=166060&highlight=speaker
The main limit to finding a small speaker that will fit in the stock side speaker covers is the height of the center the speaker. The options for 3.5" speakers shown above will fit, BUT you'll need to shave about 1/8" off of the paper material around the top of the speaker to fit it inside the speaker cover. I used an Xacto knife to do most of the trimming and then a hand-held grinding stone to smooth things out. I only stabbed myself once with the knife so I wasn't too bad.
No modifications were needed to fit the 4X10 into the dash pad. This is a "Dual Voice Cone" (DVC) speaker so it is wired in like two separate speakers. This speaker fits fine under the stock center speaker cover.
The dash frame had to be cut underneath the mounting locations all of the speakers. This is necessary to fit the larger magnets that come with the modern speakers and/or the wiring terminals. I used a jig saw with a metal cutting blade to do most of the trimming followed by a Dremel with a cut-off wheel for the hard-to-reach stuff. If I had a body saw, that would've probably worked best. I think I saw some posts about people who did some cutting with their dash pad mounted, but that would've been a major PIA. If you're thinking about doing this, do it with the dash frame and dash pad out of the car.
These pictures show the difference between the speaker that was in the dash (not stock, but probably the same size) and the dash frame after cutting.