H-Pipes inherently improve low-end torque by balancing exhaust gas pressures between the the sides. Most push-rod motors can benefit by using an H-Pipe.
X-Pipes tend to blance the flow between both sides and can increase horsepower on higher revving motors. The modern modular motors see benefit from the X-Pipe design.
Larger pipes are not always benficial to exhaust/horsepower/torque. It is all a matter of flow. It is possible to inhibit flow by using too large of a pipe. I would think the 2-1/2" would be more than adequate in your application.
