I think a fair price is $100 per system, used. So if you're thinking $200 for two systems, I'd go for it.
I have done this and would do it again.
I did a write up a few years ago. You can look at it here:
http://67-72chevytrucks.com/vboard/s...d.php?t=478564
Grab the receiver/dryers from the donors but do NOT use them in your finished product. Use them to fab up everything but get a new one when ready to charge the system.
Lots of guys on here knock the stock compressors but I've had nothing but good luck with them. With the compressor off the truck, turn the center hub. If the hub turns and you hear hissing out the back, its compressing and in my experience is good to go. You just need to flush it. You flush it by pouring in AC oil, spinning it a few times, and draining the oil out. After two or three flushes, drain it good and install it.