Well,...you are in for a long wait.
Be advised:
The K member will have to be changed out to allow that 351 to mount. The transmission behind the 3.3 will not work either. There is a 7.5" rear in your car, and it will need to be changed out as well.
More than likely you'll need a different oil pan.
Since your current fuel system is carbureted, swapping the 351 over to a carburetor will be the easier path,...
If that engine runs, Leave it alone. Change out the pan, swap the intake and put a carburetor on it. In reading through this thread, I think that you're biting off more than you can chew,..but hey, you gotta learn somewhere.
I think you'd be better off finding, and getting a donor Mustang that has a complete 5.0 and it's related running gear and using that as a swap source. As much stuff as you have to change out on the current car will take a very long time unless you have about 10k sitting in a bank,...and if you do,..I'd sell the whole 79 car and buy a completed project. You can buy sweat for .50 on the dollar.