This is going to be a bit of a toughie. I can't think of any sidemarker lights that are exactly the same size and shape as the '69 Barracuda items. You'll likely wind up doing some sheetmetal modification. '70-'71 Valiant/Duster sidemarkers are likely going to be your closest appearance match to the '69 reflector-only units. '70-'71 Darts used a different sidemarker style that would require a great deal more sheetmetal work to cut into a '69 Barracuda. '72-up cars use a surface-mount sidemarker that's easiest of all to retrofit onto cars not originally equipped with sidemarkers, but I don't think it would completely cover up the recessed stamping in your '69 fenders for the '69 reflectors.
Sidemarker lights and reflectors are both very effective (together) at preventing your car getting hit. This is a worthwhile upgrade. But it's not going to be as easy as you'd like. The regulation was phased in: starting in '68, cars had to have sidemarker lights and/or reflectors. Starting in '70, they had to have sidemarker lights and reflectors.
