1. There's a sample seat outside the entrance if you want to try it out first. They recommend it so people with back problems, etc, know if they'll be able to handle the weird position.
2. If you are too large, but kinda fit, there's a chance the restraint system won't release when the ride is over, and a CM would have to come manually release you after everyone is out of the way. This happened to someone in our party and one other person that I saw. For reference, both were significantly larger than 290 lbs. It could potentially be embarrassing, but it isn't a big deal IMO.
It's an amazing ride, so I'd definitely try it!