Is here any way to replace there mounting sill.

About the only way you can fix this is if you have a buddy that does sheet metal make the piece up and weld to bed, check with a place like Long motor. Com or classic truck I think it is an dsee if they have parts like that or get another bed, they arne't that hard to change if you get the bolts off it only takes 4 guys to lift it off. And make sure to disconnect eletrical for lights.