Is the rear spacing 120mm or 126mm?

1970s Schwinn Paramounts can and do run 27" wheels. P-10s and P-15s were designed specifically for 27" clincher wheels, while the P-13 was designed for 700c tubular wheels.

But with that said, I run both on my '71 P-13. Granted, a 27 X 1 & 1/4 tire (Panaracer TG Pasela) has very little clearance, but it will work. The 27 X 1 TG Pasela has plenty of clearance. Paramounts are spaced at 120mm and the Reynolds 531 is very stiff. I'm certain a confident person can spread one to 126mm, but I am not that person. P-10s and P-15s have eyelets and P-13s do not, if that is a consideration.

I also have a '75 Schwinn Voyageur II (Panasonic made frame) which easily accepts a 126mm wheel and was designed for 27"wheels. It has eyelets.