I run a 35 HyperDrive on a 18 x 54" GatorTrax. My buddy runs the 31 HP on a 18 x 48" Lowe. You will need plenty of HP to push your big hull. Be thinking 35. MudBuddy, GoDevil (this year) and GatorTail all make similar design shorttails and all offer or will offer the big Vanguard 35.
I have beat the everloving cr*p out of my Hyper this year and it had done very well. I think the belt drive system is simple yet robust. The props will take a beating as well.
There is no comparison between driving a longtail and a shorttail. With all respect to longtails, once you try a shorttail you will never go back.
MudBuddy is coming out with new models featuring neutral this year so if that is an important feature to you wait until March.
By the way, with a hull that big, you will do considerably better with a shorttail than your longtail, but don't expect to win any speed races, especially if you put three big bubbas and all their gear in the boat.