I am gooing to get a new exhaust on my 69 Camaro and can not decide on what size exhaust tubing to use. The car has a 355 SB with 360 HP and 390 TQ. I will be changing the heads and cam to hopefully get it to just over 400 HP and 400 TQ. I have hooker long tube headers also. I was thinking of getting 2.5" all the way back, but could I go with 3" all the way back? Would 3" be to big for the way the motor is now at 360 HP, but what about when it is over 400 HP? Thanks for the help

3" is too big, even if you were making 500hp. Dual 2.5" will do the job nicely.

The math I used when choosing the right diameter for 600hp still suggested a dual of only 2.75". (Naturally I went with 3" because 2.75" is almost impossible to find without a premium cost.)
So, 2.5" should suffice at that power level.

I'd suggest an x-pipe and some quality mufflers too.

I am thinking of going with the Flowmaster Super 40's, I like the way they sound. Also is there any benefit of using an x-pipe over a h-pipe?

X pipes properly merge exhaust from both sides of the engine, and not only pressure balance, but also allow each cylinder to breathe through both mufflers.

H pipes pressure balance OK, but they don't flow anywhere near as well.