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In 78 a space saver rim was used (15 x 5) and a tire that measured 195/80/15 which gave the tire height of 27.2". When choosing a spare combination for the tires you are running you want to come close to the tire height. Tire height for a 255/60 is 27.05.
If you have the space saver rim I would suggest a 205/75/15 tire. Fits in the carrier just like the original and has a tire height of 27.1. Using the regular rim (8") you might have to make adjustments to the carrier by lowering the bolts and perhaps more tire shows then what was original. Using a 255/60 as a spare would really show a lot.
Good luck fitting a 255/60 spare. My '77 tire carrier is made for a full size tire (225/70) and the my 255/60 spare just barely fits with the carrier supports extended all the way.