I agree with JB: wet the chest plate and maybe the back of the sleeves a little and that should help it stretch a bit. If it's choking you out then the collar is too high; get your hands inside and pull it down to give yourself a bit of man cleavage then pull the slack out of the stomach area and belt it in place. Lastly I would highly recommend (again) doing a couple of sessions with people who know what they are doing. It took me a session with an experienced lifter in the gym to realise that I hadn't actually ever got the sleeves all of the way up before
You know this though, I'm just repeating it for my own amusement I think
Once you get it on and loosened a bit, 400+ is no problem for you.