I'd use a sail plane retract. The are desiged for CL mounting and some of the better ones have shocks built in. Good luck building your Duce. I saw one of Butch's fly on DF and it was a show stopper. Ram 750 would be too much. An 500 would be more than enough --residual thrust would be less as well.
One caution, the real U-2 is very cross-wind limited (15 kts). The model I saw hit a runway light and was damaged trying to land in cross winds.