isn't rugby all about endurance? how long can a d-lineman run for?
You don't just step off the football field and start playing 80 minutes with the First XV.
Start them off early in the season getting in shape with everyone else.
Early matches get them playing, say, 20 minutes with the second (or if your team has one, third) side and start working up from there. Because good clubs focus on fitness and fundamentals, by the end of the season, those big fellas can handle a half or maybe even 60 minutes. After a second season, they may be able to go a full match, and be beastly, because they have a great combination of size and explosiveness.