I have a 1979 Brat and a 1984 brat (just a parts car) and am wondering if I can use the rotors and calipers off the 84 brat on my 79 brat?

Yes you can swap them. I would swap the handbrake cables and brake backing plates too.

Thats good to know, I've been finding it hard to get new pads. So far have only been able to get NOS asbestos pads and they really make a mess on my front wheels. 

added bonus is you get rotors with venting so your brakes work cooler. first gen don't have these and warp easily. still got my set from the 84 but has bad rotor..i still have good first gen rotors but when they go bad i will make the swap too