I found an old hughes and kettner thirty lying around my grandparents' house. It sounds pretty good, I looked it up online and apparantly it has a 12" celestion in it. Could I take it out and put it my peavey valveking royal 8, if I made some room for it?

The H&K is a SS amp, my peavey is tube... I dunno if speakers have anything to do with that. I just hear that celestions are good, and I'm sure the 12" will be better than the 8" speaker thats in my peavey now...
Yamaha RGX 520FZ
Squier Affinitys Strat (customized!)
Johnson 620 Player Acoustic

Boss HM3 hyper metal
Boss CH1 super chorus
Zoom G1X

Peavey Valveking Royal 8
New: Peavey JSX Mini Collosal
As long as the impedance matches, yes. You'll have to cut a bigger hole in the Peavey, though, so it'll be an irreversible change.

Why would you do that to a vintage amp, though?