Though not sustained, we've ran a 20 in dads Jeep for a number of years. Mid 70'sMPH for multiple passes at the dunes. Round and round it would go. In 4H, the 20 seems to be able to handle some abuse. From years on the dunes, my experience has been that you can only go as fast as your suspension allows. If the dunes you're at aren't completely flat all over, your limiting factor for speed will be your suspension.
434 pump gas SBC with an NX/HVH plate system
th350/Coan 5,000 converter/Dana 300 tcase
Dana 30 front with 4.56's/Dana 44 rear with 4.88's