Make sure you leave the dipstick out if you do fit the 4.0 crank in a 2.9 bottom end, there isn't enough room for it. The crank will catch it and rip it to shreds. Dave Ward (Sparkplug) on the XROC bulletin board and XR4 Register site has the conversion done, it make it 3.7 litres but he has an Eaton supercharger as well. His dipstick does not live fitted, he has a dummy handle to plug the tube.