i have bbk equal lengths on my 5.0. 2.5 exhaust is the biggest you want to go. it would be challenging to make a 5.0 stang need more pipe size unless running a turbo.
hell my friends 406 sbc ran better with 2.5 over 3. that motor was making 600 ft/lbs 475hp off the spray. its still the only non vw bug street car i've seen pull the front wheels off the ground at a stop light.