First off you will defiantly need to seal the steel with something like POR15. With the holes in the panels lightly tap in with a hammer and wipe over with a small amount of " need it" (epoxy steel substitute).
The last thing you want is all the filler falling off, cause there will be many hours spent shaping the filler to get it looking any good. There may be areas with an inch or so thick with bog. Your not the first person to load up a panel with bog to make it look original. It's all about the prep.