Both.
The body is longer, it has space for two dynamic brake fans vs one on a GP35, and there is no 'porch' as you see on an SD40-2, which would occur if it had a longer frame matched with the same hood/cab/nose.
Frame is somewhat longer, 56'2" between coupler faces on GP35 vs 61'6" on an SD28 or SD35.
SD28 side diagram here :
https://hawkinsrails.net/lagniappe/28s/28s.html (can't speak for how accurate - it has somehow got a turbo!)
Ben
