ORIGINAL: Clay Walters
A 12 X 8 3-blade should be fairly equivalent to a 13 X 8 or 14 X 6 2-blade prop. Any of the three should be a good match to your Magnum 91 4-stroke as long as you've enough ground clearance and the 3-blade just looks good on the T-28.
Regards,
Clay
I tried a 14 x 6 two blade against a 12 x 8 three blade MAS prop on the bench. They ran within 50 rpm's of each other. They may run different rpm's at wide open throttle with the plane flying in the air though. Don't know.