you wont need a billet stator with transmission support for stock arm in dirt. you also wont need a engine stay thats bolt on like that either with all the motor mounts installed like stock and all 4 side spacers on the main mounts.. i personally think that motor stay style looks goofy but thats just me. if you want to stay with stock covers hell rock em!
the slingshot your probably going to have to ditch or remove a lot of the weight from it to get the bike to shift. depending on what kind of load you put on the motor. dirt riding and blowing tires off everywhere it probably wont be too bad.
fox makes a piggy back shock for the rear that seems to work great. only rears i have personally rode on were axis, PEP, and marvins. but marvins on drag only and they suck for anything other than that...
being that big of a motor and i'm guessing set up for tq when the bike does hook its going to be a bitch to control. id imagine a +4 will make the bike much more fun to ride. but good thing is you should be able to just tractor it around and not really need to rev the bawlzz off it just to get it to move.
of course this is all my opion based on what i found out riding in dirt with high hp motors.