3UZ-FE Exhaust Size and Volume

[PRIMRK]

Have had a bit of a problem finding what sort of diameter as well as how much muffling I will need on my 3UZ. The car is on club permits and will be on the track/trailer most the time so can be a bit louder than the norm but still need it to be subtle round the streets for the odd occasion that I take it out.

Was looking at a 2.5 inch, a cat on each bank (maybe, may make them removable for the track or none at all, not sure yet) with an x cross over and 18" glass pack mufflers before exiting both sides just in front of the rear wheel well.

Is this diameter about right and will the volume be within a reasonable range?

Any hints/tips or rules of thumb that I can apply to this or is it trial an error?

[PRIMRK]

Is a twin 2.5" too wide for the UZ family?

If I was to make a 'Y' exhaust would I use a 3" for the final diameter or would 2.5" be enough?