61:31 (g) Additional training required for operating pressurized aircraft capable of operating at high altitudes. (1) Except as provided in paragraph (g)(3) of this section, no person may act as pilot in command of a pressurized aircraft (an aircraft that has a service ceiling or maximum operating altitude, whichever is lower, above 25,000 feet MSL), unless that person has received and logged ground training from an authorized instructor and obtained an endorsement in the person's logbook or training record from an authorized instructor who certifies the person has satisfactorily accomplished the ground training...
The Navajo Chieftain isn't pressurized so you don't need the endorsement. However, if you're really intent on flying it that high, you'd better know what you're doing. It doesn't take a lot to get into a lot of trouble up there. Get the training.
Lead Sled