Nomad;
I'd like to see the fitment of those headers if ya don'tmind.
70PA;
The problem with the inexpensive headers is there design of the drivers side. 3 out of 4 tubes run under the steering linkage. This has them closer and a bit to low for most street conditions. Daily drivers or even just sunday drivers for that matter, have flattened out the 3 tubes.
While I'd bet it still performs better than exhaust manifolds, the dents become weak points and rust gets to eating the header quicker.
Mostly recomended here for headers are the Doug's and tti's at a pricey tag, but worth the price. IDK how the shorty headers will work for you.
Exhaust tube size depends on your build. In a nut shell, I'd run a 2-1/4 pipe for a stock to mild 318 or a stock 360.
2-1/2 for a well built engine, ethier size.
For a quite muffler, I did like the Jegs muffler I got with there exhaust kit. A header back kit for approx. $200 that actualy fitwell. Minor trimming involved for some people. IIRC, it was at the tail pipe most times.
My stock low comp. (79) 318 & headers w/ small Crane cam
[ame="http://www.youtube.com/watch?v=pOYMlTTecTU"]‪74 Duster Exhaust‬‏ - YouTube[/ame]
Or the Hooker Aero Chambers My 79 magnum with a stock 78 / 400 & headers
[ame="http://www.youtube.com/watch?v=q3OTEn5mrCs&feature=player_profilepage"]‪79 Magnum Exhaust‬‏ - YouTube[/ame]
Dyno Max, I thnk it was also offers a new muffler that has a spring loaded gate that under pressure opens up for a roar and high flow.
