trouble is, the "old" 6.2/6.5 Detroit Deisels are very old. GM stopped using them in the 80's or 90's. I think. Not bad in their day, but under powered compared to todays' diesels. Although they are quite torquey.
The newer 6.6 litre Duramax series of engines is a completely different ball game. But you never see those in the UK.