There is no easy way.
As you can tell from the posts, they're welded on.
If this is a parts car as you mentioned then cutting them is the quickest, best and the easiest way. Felix is "Spot On" here with the exception of "drill on the bench"... let the buyer do that
Find the original weld points and cut at least an inch in from there. It's fairly customary to leave extra metal and leave the "picking" of the panel for the buyer. Picking is the term that refers to picking the needed panel off the welded areas.
Do not waste your time picking or drilling welds. Not only is it time consuming but the buyer may need a differnt path.
Hope that helps.