I've got one of these planned as well, and this is the method I'm planning on using:
Budget 16610 Sub as the base, which will get the lugs drilled, CGs thinned, and then fitted with a bracelet I currently own. I'm going to decase the movement and remove the hands and dial. Then, I'll remove the existing dial markers, and replace them with custom stickers I have already drawn up. I'll then lume on the stickers, give the dial a spray of matte varnish to even things out, and re-lume the hands. Next, will be removing the bezel pearl, and replacing it with a Yuki pearl, which will have the same lume as the dial and hands, then finally, I'll re-assemble it all, and hopefully, I'll have a 16800, with the only flaw being the text at the base of the dial will say SWISS MADE rather than SWISS T<25. At least, that the plan Best of luck with your project
[Edit to add]I might also pick up a budget 5513 to use the domed back as a transplant to the 16610 case (assuming it fits and seals water-tight) just to keep it in the early 16800 transition, rather than the later version with the flat back.