First time I ever heard anyone wanting a tail gate instead of barn doors. Pre 83 everything's the same iirc. After 83/84 the rear sill is different as the funeral homes and ambulance companies quit using suburbans for the most part.
I have a 86 and an 89 burb and have replaced the rear sill,bulkhead whatever it's called on the 89 after I bought it because the tailgate was going to fall off due to rust. Buy a barn door Sub and don't look back,you'll be happier with it in the long run.