Most framebuilders limited the use of SLX tubing to smaller (58cm and smaller) frames, and used SP or a combination of SL and SP for bigger frames as SLX frames larger than 58cm tended to be noodly (this was before oversized tube sets). Frankly, I would wonder about the competence of a framebuilder who used SLX for a 63cm frame unless it was custom built for someone weighing less than about 160 pounds.
The diameter of 27" wheels is only 8 millimeters larger than 700c and the selection of 700c rims, tires, and pre-built wheels is much greater. Also, most carbon fiber forks are dimensioned for 700c.
Newer components are lighter than older ones, and the only compatibility issue you're likely to encounter is the rear dropout spacing. A mid-eighties frame will most likely have 126mm OLD spacing and modern rear hubs typically require 130mm spacing. You can just live with the 4mm difference or
cold-set the rear triangle for 130mm spacing.