You probably don't want to convert any "megadungeons" to 4e... I'm currently running Sabre River (which wasn't AD&D but was from the same era), and even though the encounter sites are somewhat small (~8 per dungeon crawl), they're still taking at least three 4-5 hour sessions to complete. As you might expect, this is almost entirely due to the fact that 4e combats have a "guaranteed length" of about an hour, even when you set the monster levels, XP budget, etc. below the party level. If you go with a larger dungeon and keep the original number and variety of fights, your only option would be to make some of the encounters trivial (i.e. 100% minions); otherwise you'll never get through it. (You could also mess with monster HP, which I've started to do about halfway through the mod, but I'm assuming we're talking a RAW conversion.)