The problem with not filling the pits is this. It will look good until it leaves the garage and is driven. If you don't get the frame, and components, slick it is absolutely impossible to wash it and keep it clean. A rough or pitted frame will become an eye sore very quickly and really shows under a restored car that sits high off the ground.
I have never seen body filler, All Metal, or high build primer fail on a properly prepared chassis. Any will work depending on the depth of the pits. I suspect if this is an unrestored car that a body filler over epoxy would be the best and most cost effective. Post some pictures as you go.