Frame rail options

Behind the cradle mount is probably the worst place for things to be rusted out structurally. Surprised it hasn't folded already.

I really don't have any recommendations without seeing the damage and seeing what thes caps look like.

Section of a rail on these old cars wouldn't bother me if correctly sleeved. You aren't dealing with crush zones or advanced high strength steel that gets destroyed with heat but you will need to get things cut back to good metal.