I've had heard/seen charts showing up to 25HP/25 Ft Lbs Torque increase. That being said those claims came from the manufacture of the different system and the comparison was usually against a restrictive stock exhaust system, other tuning was done, or additional components (headers) were added to the vehicle.

I’ve done H and X pipe installs over the years and my experience is pretty much like Roger indicated, while there might have been an improvement it was not enough to feel.

I understand you wanting fuel mileage and power for towing but at some point it’s going to be a compromise of one for the other, it will depend on what is more important to you.

Staying with the small block or going to a big block I would suggest you start by throw your notions about 400-450 HP away and start your figuring from a different viewpoint. Horsepower is just a number derived from torque and RPM. The higher you can wind an engine the more the HP it will have, but what good is 400HP at 6500 RPM if you’re trying to tow something at 2800 RPM where you’re only making 150HP. Torque is the measurement of force applied and is where you need to place you’re emphasis at (in the RPM range you will be operating).

As towing is an important consideration, first look at the RPM that most of your towing is taking place. That is where the engine will be working and you want it to be the most efficient at. Start looking at the combination of components make that make most torque in that RPM range.