just a touch more than 400. I will post my build info and see if anybody can figure out how much HP it should be making

if you are buying a new system anyway, i'd go 3" as they cost the same from places like tti. 440's like to breathe

I run 3" on mine with 2 1/2" flowmasters dumped in front of the axle behind a mild built 318 with a .528 cam and i like it, makes the garage floor vibrate

the only issue i have with the 3" systems is it limits your muffler choices to mostly performance mufflers, with the 2.5" system you can get quieter (but more restrictive) mufflers.
If you want performance go 3", if you want to hear the radio, the 2-1/2" may be better.

There is a huge difference in the 3" TTI with Ultraflows (best flow and quiet) than the Flow Bastards. Load rattly and not as good of flow and neighbors hate them!!

2.5 inch exhaust is fine...even to 500 hp...I have 2.5 TTI on My bee...440/030 over...6 pack with stock high performance manifolds and it works well...I have 3 inch on My 67 GTX with a 498 stroker and fitting th tailpipes and over the axel pipes is a Pain.....I have headers on the GTX and get the drone....The bee has no drone...I attribute it to headers....both have crossover pipes..one thought...the first hemi's rated at 425 hp and were grossly underrated..had less than 2.5 inch exhaust systems.

2.5" MANDREL BENT won't be much of any restriction to a car up to 500-550 HP. it'll be much quieter, too. another option is to go 3" to the mufflers and then 2.5" muffs and tails, as by then the exhaust gasses has contracted and cooled enough to not need all the volume.

a good article:

http://www.pontiacstreetperformance.com/psp/exhaust.html

pg. 2, they only lost like .05 ET going from 3" tails to 2.25" tails....

I would highly recommend an X pipe over an H pipe setup. Major difference in noise & I think a bit more power also. As said MANDREL bends no matter what system and no neckdown in the muffs.

I run 3" pipes with dumps from the headers to 3" custom mufflers into 2 1/2 tail pipes with factory like stainless tips on my '70 duster..no restrictions what so ever,car has run the same numbers through the exhaust or opening the dumps..