Hey You Got a GT,,, Wow,, that really LQQKS COOL
Nice of your Grandfather to Help you out. He sounds like a very wise man, Lots to learn on restoring cars. The Cool Factor is you own a GT. Like Gordon says these Cars are all about the Body. The Rust, and Rotten Metal on these cars is the lack of Rust Protection from the Factory, poor Mild steel, GM must of learned from Opel, and started making cars with Galvanized zinc sheetmetal.
Body Patches, and being a Good welder, try not to patch with bondo or fiberglass filler, in my opinion
I know your anxious to drive it, but as said...Saving the Body...the shell is the most important part,,
maybe time to learn a little sandblasting too..lol
the best Part of saving a GT is being a Jack of all trades, you'll see... I wouldn't park it on anymore Grassy places... to save a GT ..must be parked on black top
in my Opinion
Good Luck and keep us posted