+1 on dropping that tank...just get rid of it as it surely is full of rust (save the sending unit though).
IMHO, I'd start with the basic mechanical components that deteriorate the most over time, brakes and suspension. Inspect all the brake components and rebuild or replace as your skills allow. This includes the hard and soft lines, wheel cylinders, calipers, master cylinder, brake cables, etc. If the backing plates or stone guards are rusty, clean them up and paint, or better yet, powder coat them.
Next, I'd move to the suspension, replacing tired springs, shackles/bushings/perches, insulators, bumpers, etc., basically all the rubber stuff. then look at the ball joints and tie rod ends and, if any doubt, replace as well. Clean and inspect the power steering unit for wear, replace seals, etc.
From there, I'd pull the axles and drop the center section and check the axle bearings and seals, carrier bearings, pinion bearings, backlash, etc. This will also give you an opportunity to clean any goo and sediment from the axle housing.
Next up, I'd pull the engine and tranny, degrease them and do a thorough inspection, replacing any worn or broken parts, installing new seals and gaskets, etc.
Then, I'd replace the heater core, re-core or replace the radiator (making sure the integral tranny cooler is flushed clean) and reinstall with new hoses.
I'd then install a new fuel tank and, if needed, lines, and inspect the wiring harness for damage and condition and repair or replace as needed. Lastly, I'd "borrow" a battery or buy a cheap used one, and make sure everything is operating properly. When satisfied, I'd change the oil and send it off to the body shop.
Why a cheap battery? Depending on how long it takes at the body shop and my past experiences, when they finish with it the battery will be junk and then you can invest in a nice AGM unit like an Optima.
Now, I'm an old school, frugal kind of guy. When I'm working on something I make 3 piles.... one of stuff that I'm definitely going to reuse, another of stuff that is of questionable condition but may have to be reconditioned and reused or discarded, and the last is the "junk" pile, but I don't actually throw something away until I have the replacement in hand.
During the process of mechanical restoration you can make your own decisions about what "improvements" or modifications you may want to incorporate. There are literally THOUSANDS of different things you can do that have been discussed here from the "Arning Drop" to spherical roller bearing suspension components to different rear gear ratios. These are all variable based on your desires.
Finally, what you end up with is your business and you might want to make it a GT-350 clone or other restomod, etc. I'd just like to say that what you have appears to be a nice, straight and relatively unmolested (except for the front apron) A-code, GT 2+2 with a couple nice options (PS and AT) in a desirable color that would be a real shame to cut up any more than necessary.
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