Panaracer does make 700c and 26" tires that can run at 50 psi, but every tire on the website that is rated to run at "around 50 psi", has a weight of at least 400 grams and a diameter of at least 1.5 inches, not the 300 grams of your 27 x 1 1/4 panaracer wire bead "skinwall" racing tires:
http://www.panaracer.com/
(re: AH261-LX-17, AH265-LX-17, AW737-LX-17). I was around in the 1970's and anything under 330 grams for a 27" tire was a racing tire, not a touring tire ~ in terms of product you are buying "Schwinn Super Record" or "Specialized Turbo", not "Squishy Tourguard" when you buy a Panaracer 27" tire!
- Don Gillies
San Diego, CA