The 225 V6 is almost perfectly matched to the early CJ5. High power to weight ratio, great engine. If you can't go over 50 with the OD, something is wrong. The 225 I had in my 71 would let me cruise all day long at 70+ mph and only about half throttle...
Do you know what ratio gears are in your axles? What size tires are you running?
Do you have a tach? It may be that your engine isn't winding as high as you think.
I suggest you visit www.earlycj5.com
The members there are experienced with CJ5s from 1955 to 1975, especially focused on the Dauntless 225 V6 and the Hurricane I4 engines. It's a family-friendly forum, all the guys are helpful and courteous.
See you there!!