Could I swap the power windows from my 82 Grand Prix into my 79 EL?

Yes, everything will swap over, using the original Elky glass. You'll need the entire harness as well, the relays are behind the passenger side kick panel and the wirng runs along under the dash along the top.

You can also buy a kit from A1-Electric for that Elky and they have some really nice switches that illuminate when the lights are on and not just one type. Of course if you are going for that Factiory Look then you'd want the GM type switches.

Anyway just an FYI

http://www.a1electric.com/catalog.htm
Window Kits for Elkys
http://www.a1electric.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=AEOS&Product_Code=GM19-KC

Switches:

http://www.a1electric.com/images6/4990-10-431.jpg
http://www.a1electric.com/images6/4990-10-201.jpg