I just purchased a new set of cranes for my 454. It has the cast finned 420 tyoe valve covers. Ive seen that some of the bbc's with rollers have the rocker corners shaved for clearance. Is this needed with the 420 covers? if so, which ones need to be cut, and generally how much?

My 454/420's have the same valve covers and Crane Gold roller rockers with no issues. I assume they are original.

It's intake rockers 1, 8. You don't have to grind that much, round off the corners. If you want, look at some pictures at Summit, or JEGS. The last set I bought they came already done.
I don't think they will fit under stock valve covers. If they have the oil drippers they might need to be ground off also. I would just buy a new set of Tall valve covers and be done with it.

The new Crane Gold rockers are different and rounded near the pushrod, so you should not need to grind them at all.

I think I have the same valve covers, and yes, two are ground and I have no clearance issues. This is the way it came from Merc Racing.

The new gold race rockers should not need to be ground at all. They just changed the design slightly a couple years ago.