70 340 4-speed dart - talk me off the ledge

IMO, if you're going to do the car,
Do it right! Get the full frame rail from member "crackedback", and repair the frame the correct way! If you are saving the car, and want it to be worthy, don't cut corners!

With that said, I would also pull it into the shop and start dismantling it and taking stock of what you have before making any decision about its future! It doesn't cost a cent to take the seats out, clean the entire interior, inspect the floor pans, and make a list of the needed sheet metal! Same with the trunk, and engine bay! The engine would have to come out anyway to do a frame rail, and won't hurt a sale if you end up not keeping it! Get it right down to the point where you can inspect every aspect of the car, but not so far that you can't turn back! Do everything you can that's free, make a list of needed parts as you go, then when you get to the point where you can see everything it needs, and how much $$$ the needed metal costs, then decide if you can afford it, and if your heart is in the job! Then jump, one way or the other!! Good luck brother!!!