AFAIK, all the CJs had the same width frame up front. The later ones got wider in the rear only. Not sure about YJs. The bigger the tire, the less room you have between the tire and frame for steering, the wider axle you need for full steering capability. If this is going to remain street legal, check with local tire stick-out laws before you commit to anything. If you go to a D60 and plan on using leaf springs, the sheer size (width) of the differential will create issues with driveshaft alignment. To keep enough room next to the casting for a leaf spring, the pinion gets placed closer to the center of the jeep. Be aware of this when planning. Transmission and oil pan clearances could become an issue.