Stay away from all that lead on the lead sled. That energy has to go somewhere and you could just end up cracking you buttstock. IMO if you can't handle the recoil you should not have gotten a big bore. I know you never said you couldn't. And I know sighting in a big bore off the bench is no fun. But when I do, I usually use a Limbsaver slip on recoil pad. Under hunting conditions the recoil seems now where near as bad.
I long for the day when I have enough cash to get me a SxS in 375 H&H or 416 Rigby.