It is very unlikely you are going to get 1000-1500 gph with just two 3/4" returns no matter what pump you get. The head losses are going to be a lot more than 5 ft with that piping at that flow rate. I would not expect anymore than 600-700 gph unless you get a really high head pressure rated pump. I would just accept a 4-5X turnover rate with the two 3/4" returns you have. To achieve the 1000-1500 gph, the returns really need to be at least 1". You would need to swage up to 1-1/4" at the discharge of the pump and then run two 1" leads to the return ports. You generally should go up at least one pipe size from the pump discharge port. Never reduce down in size. Pumps are designed with high velocity discharge nozzles to develop head. They are not the most economical size for the return pipe.