wheels after crossmember swap?

[63whoopie]

Ok, it's lesson time again; I need to be taught a few basics.

I am looking into doing a crossmember swap from a pre- 1/2 ton '87 onto my '63 C10 to give me discs and the 5x5 bolt pattern. I see 5x5 advertised on wheels that say BOP (Buick, Olds, & Pontiac)... does that mean this will fit a truck hub? I also see 5x4.5 advertised for Chevy's- is that a car lug pattern?
Also, can someone explain the "unilug" phenomenon to me? Best I understand, one size fit's more than a few hubs w/ some sort of lug nut or special shim; dosen't sound too terribly safe...
(I am not even going to waste your time with backspacing and offset and all.... I'm looking it up online.)

Thanks in advance..