If you put 78 carbs on an 81 they would work,,but I think your bike has a vacuum petcock,,there will be no port to hook the vacuum line to on the 78 carbs,,you could work around this , but is something to be aware of.
Easier to use the 81 carbs on a 78 bike as all you have to do is block the port on the carbs.
I believe you will also lose the accelerator pump if you swap to 78 carbs. I think the carbs we worked on yesterday are far from needing to be replaced. They just need a little tweaking. As Allan mentioned in your other thread, the holes in your venturi should be able to be covered by the butterflies with the idle speed screw backed off. I would pursue that angle first.
Actually I'm just thinking of getting the '78 carbs for the mixture adjustment screw that is broken on mine. If they will swap it is cheaper to buy the '78 carbs then to buy the screw from bikebandit.com; plus then I have an extra set of carbs if I ever need them for anything else.