65' Dodge custom 880

Wow. A tank which doubles as a floorpan would be cool. Almost like building a unibody. Seriously, are you talking modern Mustang tanks are cheap? I can't imagine that vintage ones would be

 

Well a vintage mustang tank is around $100 give or take. They also doubled as the trunk floor. Gas tank restoration around here is $250 not including repair of holes or cracks.

 

The only way to restore that tank you showed us would be to melt it down and then roll it back out as sheet steel.
Vintage Mustangs must have had pretty good tanks, for having survived the rest of the vehicle

 

Up until 1971 Ford's Fairlane & Torino mid size cars also used the tank for the floor, the bottoms may rot out but the tops would still be fine.
As for the Stock saddle tank I'd source an old GM pickup saddle tank and cut & fit a corner section to the wagon tank then cut out the rusted section and weld in the new fitted piece.