RE: 2 blade to 3blade prop
I have not tried to replace a 12x8 but here are some conversions I have used:
10x4 2 blade= 8x6 3 blade
11x6 2 blade= 9x7 3 blade
12x6 2 blade= 11x7 3 blade
The 11x7 3 blade loaded more than the 12x6. Maybe the 11x7 would work for you?