Reset Computer in 96 Mustang GT?

[9646GT]

Im still waiting on the ecm from a salvage yard..if its not here tomorrow im gonna grab one at the ford dealer although its double the price.

I have a couple questions for you guys with 96GT's

First one is I want to totally upgrade the exhaust from headers all the way back. I cant get anyone to do the offroad type cause its not legal..lol. I wanna go with a highflow cat system.Would appreciate a push in right direction im more interested in performance than I am sound.

Second one: 3:73 gears....how hard are these to install and what kinda performance increase would I achieve? They seem to be affordable but I bet it would be expensive to install. Good thing a local shop owes me a favor Thanks in advance guys.