My super 88 really needs some new rubber. what size was stock?
Right now there's L78-15 which convert to a 255 70R15. That seems too large for stock?
I'm thinking 7.60-15 which would be a 225-75R15
sound about right?

 

215 to 225 either will work fine.

 

factory 88s take 710-15, or 760-15 (215/75r15)
and S88 and 98s take 760-15 or 800-15 (225/75r15)
I'm running G78-15 because they're a little smaller diameter and wider than 710s
also has a 3 1/4 white wall which was my selling point

 

255-70 is huge. Thats the size I used to upgrade full size trucks when customers wanted a street tread bigger than stock.