If it got hot enough to melt I think you fell into a volcano.
I've seen the expansion tanks crack and warp and the seals pop, but never melt. Most expansion tanks on radiators are plastic. Most good replacement radiators are all metal - usually copper. I've paid upwards of 200 bucks for do it myself copper ones. Contour installation is a mother bear because it is so close to the engine. Fans need to come off from below along with transmission cooler (for ATX) lines and coolant hoses. Bought complete pumps for about 50 bucks - included back section and bypass hoses.
My pump went into a thousand pieces years ago - never had an overheating problem after flushing out the pieces with a garden hose before closing everthing up.
