2.5" in to 3" Y-Pipe

I have a mild build 360 with magnum heads. The headers I'm looking at have a 2.5" outlet. I'm on a tight budget, so what would happen if I used a single exhaust system after the headers?

I was looking at running the 2.5"s into a 3" outlet Y on back. Also there would be an electric exhaust cutout after the Y. What would the affects be?

What would the car sound like running a wide open Y pipe?

If there isn't a good idea, are there any ways to do it properly, or close to at least, with only 1 cutout and still sound good?