I'm new to this period-correct propensity -- wouldn't period-correct '73 P-13 clincher rims be 27-inchers? Is that a road you want to go down? During it's renovation (I fear I can't call it a restoration) in the late 70's, we put long reach "Schwinn Approved" center pulls on the '68 P-15, and it's happy with 700c's. It may not be period-correct; however, tire availability is not a problem.
Pete Imandt
Ramona CA