Is my exhaust messed up?
 

I just got my RB Street Header and Presilencer installed in u

Sorry about that, I put my presilencer and Streetable header on, and after the preesilencer theres the gasket, and then it splits into an output of 2 pipes into a muffler with 2 inlets and 2 outlets. Do all the exhaust systems have this? I want to put a Magna flow in it. Should i just put like a 3" pipe after the presilencer into a one input muffler, or put keep the duals and put them through a dual muffler?

Nope. The muffler you have is for an 82ish car. The pipes had to be made to split to fit the muffler. If your gonna replace the muffler, the RB muffler will bolt on and work/sound good. If your set on the magna, just remove the split pipe (hell I'd buy it off ya) and have a pipe made to go from the presilencer to the magna.

the 79-83 cars had the "dual pipe" exhaust. like Rx7carl said, if you are set on the magnaflow, you'll have to get a custom pipe made to adapt from the collector to the muffler.

Would i get more horsepower using a RB muffler, or using a Racing Borla or Magnaflow with a custom pipe after the presilencer?

I heard the RB's are specifically designed for rotary's giving max output potential. So they say.

they are designed as so. it really is the only exhaust that will hold up consistantly to the rotary's heat output.
by far the absolute best system on the market!

The '81-'82 cars used the 2-inlet muffler, although to be fair, the muffler itself really only has one inlet. The two pipes are collected before they enter the muffler proper.
The other years have a single pipe muffler inlet, a short pipe for the '79-'80 cars, and a longer pipe for the '83-'85 cars.

Oh, and 3" sounds a bit big, if you're running a stock port motor. 2.5" would be better.