8 or 16 ohm speakers

I don't really know how to word this, but i'll try. I have a Crate cab with crappy SLM speakers now, i plan on changing them out. I have a Peavey Ultra Plus head that has a selector switch on the back for 4, 8 or 16 ohm. My plan is to get 4 Celestion g12t-75s, what ohm should i get?

-cool amp, I have a soft spot for all the old Peavey hi gainer's,

-assuming your cab has a stereo/mono switch-(& 2 input jacks)-you could use all 8ohm or all 16ohm speakers.

-the easiest way to go is replace all the speakers with the same ohm rating as what's in it, one at a time, series/parallel.

-I prefer all 8ohm speaker's, a pair per jack in series, rip out any sort of stereo/mono tone sucking, transformer melting, amp killing pcb switching assembly and just utilize the 2 input jacks, 1 jack per 2 speakers wired in series.