To dovetail with what Vinnie said
GM used a 'staggered' perimeter bolt pattern on early V8's (265 and maybe 283) valve covers. Later Gen 1 Smallblocks (say 1960 through the early 80s) used the parallel perimeter bolt pattern. 1987 or so they went to the 'centerbolt' valvercover setup; Which is just as it sounds, the retaining bolts run down the center of the valvecover.
There are adapters to use the "later" (60s thru 80s) perimeter valvecovers on the 'centerbolt' heads. These may or may not cause interference with large body rockers, so contact the manufacturer of the adapters and ask.
I know of no way to run the offset or staggered 'covers on standard perimeter heads or the 'centerbolts'
PML makes some neat cast aluminum valvecovers; they're not cheap, but they're american and they seem to do a good job of hiding the breathers/pcv and minimizing the centerbolt setup at first glance, instead of using the adapters
PML Valve Covers, YourCovers.com