You can always put a smaller end piece on the tail pipe for more back pressure.
If you go with 2.5" you can install a smaller end piece.
But ,, if you go w/a 2.250 or smaller, you cannot just go a little bit bigger on your entire exhaust system, you would need to start all over again. = wasted money.
Just go w/a 2.5" system & like I said ,you can always go to a reduced size piece in the back/tip.
Another thing to consider is to go w/a mandrel bent system if you want the best flow.
Some muffler shops have premade up & over the rear end mandrel bent setup for V-8's but you can just use one side if you like.
Like for you,,,,, you could use a 2.5" mandrell up & over the rear end/tailpipe piece for a SBC 68-72 Nova, in stainless or mild steel
On my Syclone, my mandrel bent 3" up & over the rear end was for a Chevelle.
Just need to watch out for muffler shops the use the standard pipe bender & crush the pipes too much in the bends & in-turn will hurt the exhaust flow.
MBHD
