At six weeks they are ready to be weaned off their heat lamp and be in their coop. The general rule of thumb says:
90 to 95 the first week, reducing by 5 degrees a week (here's the part everybody seems to forget) until such time as you are down to 70 degrees, end of week 5, at which time chicks normally do not require supplemental heat.
Properly weaned off their heat lamp six week old chicks can do just fine in a draft free coop; even with nighttime temps. in the 40s or so. Keep them in their coop for about a week without access to their run. That should be long enough to imprint the coop as home in their little brains and that's where they'll want to return each night.
ETA: IMHO, heat lamps and coops don't mix. They are more of a risk than they are worth.