OBS Header-Back exhaust with Glasspacks and Cats

[atenisplyr]

Running Thorley Tri-Y's. They have 3" collectors which bolt up to 3" intermediates that run straight to the cats. I am replacing the cats and would really like to redo the pipes. (some of it is still stock)

I want to run 2.5" dual pipes.

Here's what I already have and want to reuse:

2(2.5" x 18" glasspacks)
2(2.5" in 4" out x 18" long chrome tips)

I want to use this line of catalytic converters:
http://store.summitracing.com/egnsearch.asp?N=700+4294923429+4294838974+42948676 33+322041+115+4294772990&autoview=sku
Obviously I need two of these.

Gonna need two of these as well: http://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM%2DG2990SS&N=700+4294859977+305321+115&autoview=sku

At what point should I reduce my exhaust from 3" to 2.5". Should I get 2.5" or 3" cats?

Does it matter how far the cats are placed from the O2 sensors?

Should I keep the stock flange where the cat-back system starts or just plumb it out?

What's gonna be the cheapest way to get the pipe? I don't really need to buy a full exhaust kit since I already have some parts.

There will be forced induction at some point in time. Other than an upcoming Marine intake, I don't plan on doing any internal mods until the motor blows.

[atenisplyr]

Been thinking about it, I just might try this setup.

Run 3" pipe from the headers to 3" cats. Run 3" pipe to just past the last rear O2 sensor. About 4" past the last O2, reduce the pipe size to 2.5". Run some pipe to the glasspacks then put 2.5" turndowns just past the axle.

[mgrotel]

id probably just get a cat that has a 3" inlet and a 2.5" outlet and if that brand doesnt make one id jsut put an adapter from 3 to 2.5 right after the cat

[atenisplyr]

So, this is what I'm gonna do. I couldn't decide if I wanted to go with turndowns or tips exiting out the rear, so I'm gonna do BOTH!

Run the 3" I/O cats out of the Thorley's.

Reduce to 2.5" right out of the cats.

Increase the port size on the stock flange to 2.5" so I can reuse it.

Run dual 18" 2.5" I/O "hollowed out" glasspacks. Kinda like straight pipes with a "muffler".

Install two manual cutouts coming right out of the glasspacks. These will act as turndowns if I want to open them up.

Run 2.5" pipe to the tips and call it good. Should sound pretty badass.