This is simply my opinion and is not based on any expertise, but...if the bike is not going to be a collector's item where keeping it original will affect its resale value, and you simply want to ride a nice old bike, then yeah, upgrade that sucker. Whatever it takes to keep it on the road.
My first bike build was a Raleigh Sports, frame only, that turned into quite the frankenbike by the time I was finished. However, most of the changes were upgrades rather than blatant changes...I kept it a 3-speed.