which is correct for an 1968 SS396 short or long door arm rest?
which is correct for rear seat arm rest short or long?
was this an option choice on interior selection?

if it had chevy interior short, the buick interior had the long ones

Pics of Chevy armrests w/measurements--I think that the SS models all came with the Deluxe interior in 68. If I have time later I'll check on that. Some cars came with the Buick interior due to a supplier's strike, they have a -178 code on the trim tag. Note the length of the front armrest backing plate--68's are longer than the other years.
BillL

I beleive ARK68SS is correct. I just looked at a picture of a 68 SS door panel, the front of the armrest starts about where the SS396 door badge begins. That could be a good reference of help, the pictures that ARK posted coincide with that.