Look at how I did mine. Make some alum. blocks mounted in between that channel and draw up some brackets to screw to the blocks. Several different ways you could go.
That was for one set up for an LS with no fans and no trans. cooler fittings.
EDIT... I just rechecked their website and I must of got quoted WITH the fan, $800 is in the ballpark with the others.
I talked to the guys at Mosley and would of given them a try, but at @ $1500 verses @$500 for the Griffin 🤷♂️. I'm sure the Mosley is a good radiator, just not three times better for my application.
You're looking at 24" to 26" across and @ 19" high. I just bought a Griffin which looks sturdy and made in the USA, Ron Davis makes top notch ones. I would talk to the actual manufacturer in order to get what you need. J Mack went through a lot of trial and error before finding the right...
Sorry to see the rust, but it's good you're going to deal with it before a big money drop into drivetrain. Unfortunately there aren't patch panels for those rust areas, you'll have to make them by hand. I would start to look for a trusted body man and go over the whole body getting off the...