Had this discussion about a 69 3 door Burban too....
So, if I were to do it, I'd remove the entire roof from the RRC and make an internal cage. Hoops at the B pillar, C Pillar and cage them in. Then, now the weight will be okay, measure the Vanagon top. It is not much different in width it appears, but the length and see if you can mate it to the RRC roof. If you can, and it looks decent, then just cut out the section needed to get upstairs and then rebolt it down, making sure the roof rests on that internal cage for support.
Otherwise, save for you Westie Synchro, you drive the RRC and your better half can drive the Synchro, or vice versa.......or stick to a roof top tent.