I would say 285-300hp with stock smogger gm heads or 340 to 350hp with a good set of heads. (vortec, afr, dart, edlebrock.....)
More Hp is not the most important thing here. More torque is what you should be after. (if you have a highway type gear).
The 260H cam is about the biggest you should use with a highway gear in a 350.