i wouldnt fire it up right away. likely to need filter caps and paper capacitors replaced for safety reasons. if you fire it up and one of them blows, it could spell death for the radio, and cost much more than its worth to fix.
on to the deets:
its a nice radio, pretty good performer.
its a model K-731
not alot of value on this radio, basically worth whatever someone would want to pay for it. in good playing shape ive seen it listed for 35 bucks. i would call it an antique as it is still tubed chassis, and over 50 years old. the original packing is good to help with value, though.
yours looks to be in very nice shape, as one would expect being packed in its box for all this time. i would get it checked out and keep it as a display piece in the living room, or dont spend any money and just use it as decoration.
keep me updated on what you decide to do with it. cool lookin piece!
edit: i missed where you said you already plugged it in and turned it on LOL. well the same still applies for future reference. seems you got lucky with this one. the Zeniths seem to be able to handle long times of non use as both of mine from that same era fired up and play fine without any replacements
got the CD changer in the car loaded full with 'em 
