First, I have and daily drive a Diesel coupe and have had it for 8 years. A superb car and I don't want to drive anything else but .....

I would say your car is too far gone to bring it back. You need so much that it would be far less expensive to buy the best one you can find and start from there. There will still be plenty to do to make it a daily driver you can rely on and be proud to be seen in.

The fact that the floors are rusted through says a lot about the rest of the car. An engine replacement or rebuild is small stuff compared to structural damage. Add to that a new interior, suspension and steering rebuilding. When you are finished, you will have a gas coupe with a rare but fuel-hungry 280 engine. I love that dohc engine but it gobbles fuel like crazy. They are good in an SL but in a big body coupe, not so much.

Unless you are a home-equipped expert body man and are bringing it back as an expensive hobby to simply spend time wrenching on, I would part it out. It is quite possible to part that car and actually make some decent money on it since coupes are rare.

I overpaid on mine eight years ago at $11k but it was a desert dweller with no rust, a replaced Diesel engine, a fully replaced interior and a fresh paint job. I will never let it go but it still has been expensive and time-consuming maintaining it in excellent condition.