tire fitment

[78 Bird]

Hi, I have a 78 Firebird with stock 15x8 snowflakes and I was looking on mickey thompsons site and saw their sportsman R/S tires that are somewhere around 12 inches wide, but they say they will fit an 8 inch rim, will this work or will it be too much for the rim? I've never seen a 12 inch tire that will fit an 8 inch rim, thanks