it can be done in the car but it's definitely easier to do it out of the car. If you're putting in a roller cam I wouldn't even try it in the car as the best/easiest way to check end play is to pop the cam plug out and use a dial indicator.
edit: also, if you're going to do the cam I would pound some new cam bearings in at the least. I would actually at least check all the mains/rods as well but that's just me.