It would have been easier and less expensive to buy a whole bike, new or used. You have a 40 year-old frame built for 40- year-old tech. Nowadays, there are so many options with how you could build this that you need a guide. 8-, 9-, 10-, or 11-speed cassettes on a 126 mm rear axle? Down-tube levers or dual-control levers? Handlebars that will work with dual control levers and a quill stem? 25.4 mm or 26.0 mm? Where to buy an appropriate quill stem? And what about tire sizes and rim widths? Can I run 30 mm tires in a 40 year-old road frame? Saddles and seatposts? Road or MTB pedals, and then, which ones? Crank arm length? Shimano, SRAM, or Campagnolo? Triple chainrings or double, and what about 1x? Clamp or tab mount front derailleur? I recommend studying the Park Tools Blue Book of bike maintenance as a reference, but it sounds as if you need more of a tutorial or an in-person guild. Is there anything like a bicycle repair co-op in your community?