I think there may be a little more to it than that - because the hub/drum assembly comes off as a unit, and the dually hubs stick out farther than the SRW ones meaning the axleshafts will be a different length. (Being a full-floater axle the hubs don't just slide off the wheel studs, rather the hub and drum are a complete assembly held onto the spindle by a locknut. On the full-floater axle the shaft only transmits power, the axle tube carries all the weight)
My '75 was orginanlly built as an SRW 1-ton, I put the duallies on it by swapping in a complete rear axle. IMO that's your better plan if you intend to go that route.
Remember too that the 3/4-ton frame is quite a bit different than the 1-ton unit: rear shocks mount outboard on the 3/4, inboard on the 1-ton, and the 3/4 ton frame is lighter - the frame channel is 6" deep as opposed to 8" on the 1-ton.
So unless the duallies are for "looks" I don't really see any benefit to having them on a 3/4 ton.
About five years ago my father wanted to put dulies on his 86 3/4 ton and the only realistic option would have been to swap the whole axle for a 3/4 ton axle with smaller brakes or to go to a one ton rearend. He decided to leave it alone since it was impossible to find a good rearend with the small brakes and the same gear ratio.
Unless just for looks don't waste your time or money. I bought a srw truck that had been converted(at the time I didn't realize it, was young)to a duelly with a dump box. Gvw was 9,000, went to quary one day to pick up stone, empty truck weighed 7,800. I was always overweight, no matter how much I didn't want to be.
Luckily I never got stopped. If a screening plant didn't listen, I was in deep, one guy loaded it so heavy the hitch was only like 4" off the ground. Hope this helps. Oh & stopping wasn't to much fun, even though all brakes were new & properly adjusted.