The engine itself may have problems if you try a larger prop. Anything from erratic idle to very low top end.
As far as the prop goes, the slower your motor turns the better a multi blade prop will work! Thrust is created when a prop BITES the air. On our high rpm small engines, a 2 blade works wonders because there is sufficient time between blades. Now when you go to multi blade, time between blade is cut down and the blade is trying to push previously cut air(in essence cutting the vortex!). This is highly innefficient! More blades, even less efficient. So if you can get your motor down to about 4,800-5,500 rpm allowing the air to recover between prop passes you will regain efficiency.
Byron has a gear reduction just for this purpose on their big planes and also the reason you see 4 blade static props but pilots then put 2 blades on to fly.
You are also going to be handicapped using a Magnum!
Use a 2 or 3 blade and save the 4 blade for static use.