225 exhaust advice needed

Hi guys/gals

I've got a delema. I've got a 68 4dr that's got the tower of pwer under the hood. and I've aquired the super six with alm. intake. I need to upgrade exhaust and I'm curious to know how much of a power gain is there with oem manifold v.s. duals. I'm not thinking about any other headwork or cam at this point as I haven't decided to go V8 or not

If duals are used, what size? 2 1/4, and could this be used with a 318or340?

my goal: is to lay some rubber when I drop the hammer, and sound decent

thanks