Mounting tire on rim at home

[truckin 79]

Anyone have any tips or tricks to mounting a tire to a loose rim? I recently put new tires on my truck but kept one old tire to use as a spare. The spare I have now is a bald donut not worthy of being called a tire. I finally found a 15x8 wagon wheel for my spare. Well two places I took the rim and tire to said they can not mount because tire is 22 years old. FYI this tire does not look like a 22 year old tire, plenty of tread and and not dry rotted. I knew the old tires were old but not that old, would still make a good emergency spare. Anyway I tried soapy water and pry bar but did not make a lot head way mounting tire on rim.