As Fred said, the Frame is heavier, as are the springs and shocks (and the way they are mounted is slightly different).
The 1/2 ton may not have as beefy an engine, but that's easy enough to swap out.
The rear end is geared differently, and the brakes are usually larger as well.
3/4 tons may have a longer bed... you may want to be careful with that.
Aside from that, you should be okay with a 1/2 ton to 3/4 ton frame swap. There'll be some finagling when you mount the body, but it shouldn't be too bad.
