Yeah, that used to be a common thing to do. I believe there are still companies making kits to put a single carb on, as the original Keihins can be finicky, a lot of manuals actually have INCORRECT directions, and most parts kits are crap. The bad part is that these conversions almost always hurt the performance. You might send him to
Randakk's Cycle Shakk® if he doesn't already know about it. He's the GL GURU, and has excellent tech articles and sells a high quality rebuild kit. When I did my carbs, I discovered they had been done wrong by the shop I had do them 15 years ago, and once I got them put together right, and synced, the bike runs better than it ever ran previously. Starts and runs like it was new.
Timing is equally as important, and the points wear out at a pretty good clip. Syncing the carbs and setting the timing was an all day job. You really learn to appreciate computer controls, and the ability to just plug in a cable and have complete control over everything.
They're really cool bikes, and were about as shocking and different in their time as the Rocket is now. 80 HP was mind-blowing. In 1975 when the GL1000 came out, 1000cc's was a huge engine. Remember the fastest bike of the time was the CB750, and the KZ1000 didn't come out until 77. They were also similar to the rocket in that they were designed as a large powerful bike, but the sales really came from the American touring market, so they detuned the engine in '77 to produce more low-end torque, and over time it morphed into what it is today. If it wasn't for the Rocket, I think I would've wound up with one of the older Valkyries. In fact on my desk, I have two model motorcycles....a Rocket III and a (Pre-'Rune') Valkyrie.
Of course now days 80 HP in a 700 lb bike is not as amazing. Which is why Randall Washington (aka Randakk) supercharged his....