Decision: 5x4 (SBP) or 5x4.5?

Performancewise, it all depends on how you intend to use the car.
The biggest advantage of disk is cool off during repeated high speed or steep downhill use.

Weigh the various factors before committing.
Stock steel wheels will fit over either factory fixed caliper or '73 up.
Going wider on '69 Barracuda will use up precious clearance as mentioned by another poster. So you'll have to be more careful with backspacing and tire size choices.
Decide what you're doing for the rear axle to convert it to big bolt pattern.

Wheel choices have increased for SBP in the past 10 years or so. Still nowhere near the availability for 1/2", 5 on 4.5.
So maybe see if the wheels you want are only a BBP, then go from there.