You can probably buy a bike made out of Reynolds 531 or Tange
#1 or
#2 or Ishiwata 022 or 019 for the cost of fixing up the Grand Prix, although you might have to put a little money into a new purchase. Any of those steels should give you a better ride. Depending on the year it was manufactured, your GP may have steel wheels, and I think you'd need to get new, wheels with aluminum rims if you want to stop in wet weather. That could be a big expense.
Still, on one ride a couple of years ago, a guy on an early '70s GP passed me and most of the other people who passed me (and whom I could still see on the road ahead) as if we were standing still. But ... if you ride a bike made of high-end steel, I don't think you'll be sorry.