I found that 13x6 was an good size for the 70 Surpass. A 12x4 is a bit light.

You do NOT want the engine revving past 9500 on the ground, because it might exceed 10,000 once it unloads upstairs. Going past 10,000 is an almost sure way to float the valves and shorten the engine's life. An indicator of this is if the engine seems to "surge" in straight and level flight.

Get an accurate tach. I think you'll find you need more prop.