Exhaust size?

I think you would be fine with the 2 1/4 for your use and you will still see a gain with the headers. 3" would be overkill for your mild 340.

You have 2 1/4 now and the cost of adapting it to your headers will be very low. Try it out, you will probably be happy but if not you can change to 2 1/2 down the road and not be out anything.