there is a popular sentence which is the same meaning"山无棱，天地合，才敢与君绝“。^_^

also we can say 我想永远和你在一起。

if you think as long as possible=forever

this expression is more confortable.

我想一辈子和你在一起。

我想和你磕终身！  This is an awesome expression.

Yes,that is completely correct.

You can replace “长” with “久” as well.“长” can refer to length of an object or duration of time.But “久” can only refer to the latter,and is more commonly used.