Best way to separate pinch welds

I have a similar situation, only I can't get to the "scrap" side due to little clearance. I have a set of cut off truck bed sides and I can't get in under the upper lip to grind that portion away...... was gonna go at them with something like a Blair rotabroach on both the donor and recipient..... unless someone has a better way to do this.
Id liek to seperate them as if "everything" was wanting to be reused. bot on the bed I am rebuilding and on the inner bedsides on the cutoffs.....