Let's talk a little bit about Mikuni needles. They come in different series and the first number in their code denotes which one they are. Each series falls in a 10mm length range. 4 series needles can be between 40 and 50mm long, 5 series between 50 and 60mm, 6 series between 60 and 70mm, etc. Here's a pic of the various needles used in the BS38s. '77 and older carbs with the old style slides use shorter 4 series needles. The '78-'79 carbs, due to their redesigned slide that mounts the needle higher, use a longer 5 series needle .....
Notice the big difference in length, probably a good 6mm, between the 4M1 you're trying to use and the 5O2 that's supposed to go in that later slide. Now, considering the fact that needle clip positions change the needle height by 1mm, that's a huge difference in length. It would equate to running the needle with it's clip moved 5 or 6 slots. Just moving the needle one clip slot can make a big difference in how the bike runs so just imagine what moving it 5 or 6 might do, lol.
Now let's talk about the needle and needle jet combos used. Each needle was paired with a certain size needle jet, and the rest of that carb's jetting was tailored to work with that. The 4M1 needle was mated with a Z-8 needle jet. This was the largest (richest) used in any of the 650 BS38s. Because of this rich midrange, the mains used were quite small, 122.5 in the States and as small as 117.5 in some European countries. By comparison, the 5O2 needle got mated with a Z-2 needle jet, the smallest (leanest) put in any of the BS38s. Because of this leaner midrange, the '78-'79 carb set got 135 mains, the largest put in any 650 carb set.
So, where am I going with all this? Well, to get that '78-'79 slide to work in the older carb body, I think you're going to have to use the 5O2 needle for sure, but you may also need to go with the rest of the '78-'79 jetting specs - Z-2 needle jet, 135 mains, and 27.5 pilots. At least to start anyway, then re-jet for any mods basing that on having the '78-'79 jet sizes as stock.
Needle taper may come into play here too. The 4M1 and 5O2 needles appear to have a similar taper but the tip of the 4M1 is fatter. That means using the smaller tipped 5O2 in the Z-8 needle jet would probably give a richer midrange than the 4M1 would. I'm not saying this wouldn't work and in fact, it may be beneficial with mods. You'd just have to try it and see.