I would start out shifting it around 6000 RPM for several runs and then try 6500, if it runs quicker and faster at 6500 the try 7000 RPM and go from there. I look at all th enumbers on the time slip, sometimes over revvingit in 1st and 2nd gear make the car faster, some times not Not all tach reads exactly the same also
The big head, big cam motors high compression race motors like different shift points than other combinations. Do all your testing based on the time slips and spark plugs