what I've heard is its not about the horse power you get from the simple raise in displacement, rather its the potential you are possibly unlocking by increasing your displacement, so I've heard, and I can be wrong
With stock replacement items i.e., h/c/i, you have very little hp gain. With shorter pistons, longer rods and rod bolt reliefs cut on the block with the same 306, and upgraded h/c/i, you have a torque monster 347.
Either way, it's better than the old worn out motor.