I met a guy on the weekend who plays pro rugby as a front row. He's 6'1, 130kgs/284lbs. He's reasonably lean (for a guy that size) too, hasn't touched gear, and is 24 years old. So I'd say you need to put on about 100lbs of useful mass, without much of it coming from fat gain, if you want the actual physique of a rugby hooker.
The thing I'd caution against, having stood toe-to-toe as it were wth one of these guys, is thinking you can achieve that physique unless you are born for it. You say you are quite big boned, but you're still 100lbs/45kgs lighter naturally than a guy with the physique you want. Without trying to discourage you from pursuing your goal, you should be aware just how massive these guys are naturally. They could never touch a weight and they'd still outweight you by 70lbs or so, probably by the time they are 18. I'm all for the whole 'chase your dreams, don't let anybody tell you otherwise, only you can determine how far you can go' etc, but a degree of realism will help guard against making poor choices with your diet and training. These guys really are naturally far, far bigger than even more 'big-boned' human beings.
There was an article on the bbc the other day that showed what one of the Welsh forwards ate everyday to maintain his weight. It was about 8000 calories. And he was 'only' 18 stone.