The problem is that 4 3/4 wheels don't have a large enough center to accept the truck rotor hub.
There are a couple options though.
The back is easy. Take the hubs off and redrill them. Take the axles out and have a machine shop drill them for you. And machine the center of the axle down a bit. Also to accept different 4 3/4 wheels. I have done this.
Classic parts of America sells a cool conversion kit for this. But you need 64-70 front spindles. Which are easy to get. Here is a link to it.
http://www.classicparts.com/1964-70-...ctinfo/71-977/
I have personally used this kit. I recommend it. Very easy to install.
I wanted to put Chevelle wheels on my 69 short step but ran into the same issue. This kit fixed the problem.
Hope this helps you out.