Compatible parts between 2001 V6 Double Cab and 2003 4Cyl Ext Cab

I own the 2001 4WD V6 Automatic Double Cab (VIN 5TEHN72N51Z748007) with 248K+ miles and have a lead on a $1500 2003 4WD 4Cyl Automatic Extended Cab with a blown engine(VIN 5TEWM72N53Z233425) that supposedly only has 83K miles. I tried AI to see a list of interchangeable parts but it wasn't helpful. Partsouq.com has schematic drawings with part numbers that don't match for the two different trucks that seem to have the identical automatic shifter, for instance. I'm wondering how much is interchangeable as far as the chassis goes, frame, axles, diffs, brake components, etc. as well as body parts. I know the beds are different, cab obviously and everything in the interior behind the front seats. Of course, engine and trans are different. I'm told it's on its second frame and that's "starting to go". Not sure what that means. We're in NH where the rust is fierce, but with that low mileage, one owner, I'm hoping to maybe get some front body parts and low mileage under carriage stuff.
Any thoughts?