Have you thought about getting an actual classic frameset from the late 80s or 90s?
Last year I built up a 1983 Guerciotti(Columbus SLX tubing) with 1991 Campagnolo Chorus components, and it cost about $1200. The frameset was NOS, and cost $500. I was lucky to find a quality frameset for $500, but there are some great deals to be had if you aren’t in a rush to complete the project.