I straightened my frame as per brets advice & it worked out well. I have worked with metals doing fabrication work & tool & die & one thing you don't want to do is throw water on it as this will make the metal brittle.
on mine the tang broke off the oil pump drive apparently because the oil pump locked up. you could spin the oil pump shaft and feel it lock up and have to turn it backward to get it to unlock. could not find anything inside it but it wiped out the rear main
it is possible there were cracks in these areas & they were cut out & butt welded in. there are some small cracks in my firewall which will have to be welded but not bad enough to have to cut out any material. good luck with your project & keep us posted on your progress
if the tank is out of the cab you should be able to see where the rivet holes were & be able to redrill them from the back side. the plates that are there are the original reinforcement plates.