How compatible is the parts from year 68 models to the 69 models?

Depends on the parts, however in general the 69 and 70 share the most in body parts as there was a restyle done on the 69 body that changed them quite a bit from the 68. GM at that time was doing restyles about every 2 years on some of the lines. It's one of the pitfalls of owning older vehicles, parts can be hard to find. Usually, the ones in salvage yards have been crushed for scrap and unless it was a real popular car, there are usually no parts support in the aftermarket for anything that wasn't used across multiple lines. What parts are you looking for? Have you tried using https://www.car-part.com/ to see if any salvage yards might have one?

Okay, engine parts shouldn't be to bad, they used the same 472 from 68-71 Fuel tanks are a bit harder. If yours is soled but rusty inside you can use a bath of acid and water to eat the rust and a pressure washer to clean out most of the crud, then use some PR-1422 or PRO-SEAL 890 tank sealer to coat the inside if the tank and keep it usable. Radiators are available and the carb can be rebuilt with a kit, or if you wanted to modernize it a bit you could bolt on one of the stand alone EFI kits. Biggest thing to worry about is that they were not set up for unleaded gas and really don't like modern oils. For the first one you pull the heads and have hardened seats installed so the valves don't erode and fail and run 93 octane due to the higher compression ratio. Stick an electronic ignition kit on it to help with igniting the fuel. The oil issue is because that engine has a flat tappet camshaft with high spring pressures and modern oils don't protect those very well and then you have cam failure. You can solve that with a roller cam kit. That takes the worry of that out of the equation, and lets you use a more modern oil. Been through all of this on my various project cars over the years.