In my experience, there's not a nickel's worth of difference in the lot of them. And don't get carried away with the "huge pipe of the month" club either. What a bunch of tripe. 2 1/4" pipe will be a God's plenty for the little 318. I would pick up a couple of glasspacks, the longest that will fit on the car and intall them with the louver openings facing the motor.
I've been running turbo mufflers on my Charger for years and years(yikes! I can truly say that!) and have been very pleased. I think that 2 1/4" pipe will be all you want. I had that size on mine until last summer when my buddy talked me into letting him put 2 1/2" pipes with flowmaster mufflers. I think the car lost some of it's umph, and it sounded like a mustang, so I put turbos back on.
I ran the Sonic Turbos on my 69 mach but that was 15 years ago. I wasn't even sure if they still made them. 'LITTLE'??? You must be a BIG BLOCK guy. Relying on the old pressure check not the Dynos its running around 370hp.Im OK with that.Not sure if it is the original block but it is a 66.THANK YOU for your input.
Pay attention on the engine design issue.It could be a Polyspherical 318,not a LA 318.The valve arrangement is totally different,thus headers could be a problem.Well,being that it is a '66 car it could be a 273 which is also a LA modern SBM.Check the front of the block,left side,just bellow deck gasket.The displacement should be stamped there.
Dynomax bullets will make that 318 sound like the unholy beast. Got a pair of those on a 93 dakota 2 1/2" pipe and hedman shorties. I know someone with a 66 with that motor and headers for that are a crapshoot unless you can find someone that can weld a set up for you. Only company I know of that makes headers for a 66 charger 318 is flowtech. I believe they have a listing on summit for them.