Your lovejoys look to be a size L, hard to say for sure with pics. The stumpgrinder attachment I own uses belts between a gear box and the grinding wheel. The belts absorb shock. I use a type L lovejoy to couple the hydraulic pump between my engine and pump on my wood splitter. Just a visual observation, but I dont believe a type L coupling will standup to the abuse of a stumpgrinder, if used to direct couple the hyd motor to the cutter wheel. There are other types of lovejoys that might handle the abuse. I dont know the type number, but there is a lovejoy spider coupling that uses two spiders encased inside a collar. The shaft ends look similar to the coupling that you are currently using, but the collar is made to accept a spider on each end and goes between the two shaft ends. There is also a chain type coupling that uses roller chain to connect the two halfs of the coupling that I believe would be a better choice to connect your hyd motor direct to the cutter wheel shaft. A collar is also available for the chain coupling to protect the roller chain from dirt and to contain grease for lubing the chain.