Sonny you only need the amount of HP that the speed of the cylinder vs.pump GPM requires..if you have a dump trailer then the cylinder speed really doesnt have to be that fast..do you already have the 8hp engine and are designing in reverse? I think the cheapest way would be a 11 or 16gpm splitter pump,it would give you fast dump speed with light brush or light material, but would have the pump kickdown for heavier material dumping at slow speed where you need hoist power..the 11gpm pumps are everywhere for about $100..even a 5hp engine will push a 11gpm 2-stage pump.