Sentimental is a good reason to keep the 73.
Here are a couple of reasons to get rid of it.
Front suspension. Pretty much a one year deal. Hard to get parts for from what I hear. the 74 and up have the Mustang II suspension, parts galore.
Manual trans isn't the best and can be hard to get parts for (this is true of all Pinto's with manuals) especially the shifter.
Hard to swap a V8 into, engine compartment is quite a bit smaller than the 74 and up. Most people end up cutting the radiator support to accomplish this.
Now, most think the pre big bumpers cars (73 and earlier) are better looking. Although you can swap the small bumpers onto the later years. Or tuck the big bumpers in. And the front end is different. Really depends on what you think is better looking.
The V6 isn't a bad engine. I had a 75 Mustang II that had that engine and auto trans. It was alright. Not a V8 sort of fun, but it was a good driver. And it would be much easier down the road to swap a V8 in if you wanted to later.
I would probably do the swap if I was in your situation, but I admit I would have a hard time getting rid of my grandpa's car. He would think I was being an idiot though.
Russ