Ouch! (To my idea of drilling. Actually the drilling probably won't be so bad, but the tapping? I wouldn't.) My Plan B might be to research DT clamp-on shifters with pipe-clamp style bands and bed it with inner tube roughed up on both sides. A tiny spot of magic marker so I could watch for slipping. (Well, probably just start with shifters I liked, the innertube and the spot. Maybe that would work and no harm done if it doesn't.)

And hearing that weight - nice! A real keeper! I have an '83 Miyata rust bucket, boldly decalled Pro Miyata. The bike I raced only 7 years of intense development better. Tubes that didn't exist yet when I was racing, Dream fit. Nearly the parts and wheel I used to race. Rust is now at bay with a circus's worth of custom mixed fingernail polish. But the ride! That bike takes me back! It's not your bike by a long shot. It will never see comfy wheels. It will never be anything but race. Yours looks like the classic and sweet Japanese "sport" bike that had such a nice ride and was/is so versatile. That I tried to copy with my first ti custom. (I got everything except if I am not super confident, it is disturbingly nervous at high speeds. Getting worse as I age and lose both strength and confidence. The good Japanese outfits would have got that right.)