I'd say a classic 55# would be good for a thin multi. Too tight and the frame will take all the impact. Plus, it's getting cooler there, yes? Tight strings in a 40 year old racket in cold weather...yikes!
On the other hand, you could just try it the way it is. THEN, if you like the general idea, restring it with the good stuff OR look for a higher-end model. By the bye, is there a country of origin on the frame...perhaps on the butt cap?
Huh, just checked the weather there. Guess you'll be indoors