If the 79 swingarm is for a drum brake, everthing from the 72 except the chain gaurd should bolt right on. With a bit of modding the chain gaurd will fit.
If the 79 is a disc brake arm then you will need to add the brake stut mount to the bottom of the swingarm.
In a stock appilcation just swapping the swing, same shocks, will lower the rear a bit.
I think the 79 swing arm is built stronger than the 72. This will improve the handling a bit.
One more thing. The axle plates on the 79 are thicker than the the 72 so you might need the later rear axle. Maybe the wheel spacers too.
Leo