all depends on your weight and which 240 kit you get some make alot more power than others. and if your skinny like me all you'll do is spin the tires and the longer the swingarm the less weight on the rear tires so less traction so you'll want to keep good tires on it and remember to shift your weight further back than normal
I've got a +3 swingarm on mine and can still pull the front wheels off the ground. I wouldn't go longer then a +4. +2, +3 or +4, any of them will work just fine.
my +4 keeps the front wheels on the ground pretty good. Keep in mind that is why you lengthen the swing arm, other wise a highly moded blaster with out and extended swinger would be pulling the front end off the ground TOO much. I would recommend a +3 or +2 as Id like to wheelie a bit easier even though I still can it takes alot of rear weight shift and Im 190 lbs.
it depends how wild its going to be ,some can get away with +4 while others run over twice that length ,one of my customers runs a +12 on his rigid rear drag blaster and it can still come up if traction is ideal