You're correct if the stripes actually have the "H.O." letters. That would NOT be correct for a 400, even with the 400HO motor.

From the posted picture I can't tell if the letters are there or not. And, I can't go buy one to check it out because my wife would kill me if I came home with another Firebird - even a Hotwheels!

But the hotwheels you could sneak in.
Or does she search you every time you come home...

I checked my red ones and green ones that have letters "H.O" on the white side stripes. I think 400's with factory H.O package is very rare that came out in 1967 and 1968 only. The Formula package came out from 1970 and up Firebirds, with factory Formula twin hood scoops not Trans Am package. In 1969 Trans Ams have factory twin long hood scoops with Ram Air package is not a formula package but that's similar to 1970 and up Firebird Formulas. I'll take some of pics of my six 1967 Firebirds 400 with H.O white side stripes then I'll post pics here for you to viewing them soon.