Six Flags Magic Mountain HELP... I really want to go but I might cancel...?

it's a hard question to answer, the issue isn't necessarily the weight, but the actual size, as the restraints have to be able to securly lock, IE rides that have inversions will require shoulder restraints while wooden coasters etc will require lap bars which have a locking mechanism (sometimes a seat belt as well) that will have to be in the same position for each car as the bar lock is shared in groups of 2. (IE 1st row of seats and 2nd row lap bar will lock into the same position.) Regardless, you should still go even if you aren't able to fit as you can still be with your friends.