318 exhaust

i have a 74 Dart with the LA 318, currently the exhaust is still the tiny single exhaust from the factory. My plan is to put a 2.5 inch duals with an x pipe and turbo mufflers exiting out the sides before the rear tires. Will this set up have any noticeable power and mpg gains? I know the exhaust will have a positive effect over stock but what numbers would i be seeing?