You don't want MAC's. Do a search. There are a number of complaints for excessive oil consumption and they have slag from poor welding inside that enters the engine. The finish sucks too...they will rust before your eyes.
Buy BBK's. Best all around fit, finish, and price. I have had them on my 96 for 7 years..they still look great, and have not caused any issues. They are less expensive than other brands as well.
I dont know about the secondaries, but the primaries size indicates how wide the diameter of piping is from each exhaust port to the collector. Usually if you are making less than 500 hp you will need to go with a 1 5/8 primary to a 2 1/2 collector. They also have 1 7/8 to 3 setups, but you need a lot of power to require those.
There are also stepped headers, which start at 1 7/8 and then step down to 1 5/8 halfway through the primary and go into a 2 1/2 header. this promotes savaging- Kooks is the only brand that I have seen make these...