Timing for spit roasting a 42lb lamb

In 'How To Grill' Steven says it should take 3 to 3 1/2 hours for spit roasting an entire 25 lb lamb. That's about 8 minutes per lb. Any suggestions on how long a 42lb lamb should take? At 8 minutes per lb that's abou 5 1/2 to 6 hours.

Thanks,
Karl Kacerek