the only mods i would do are better flowing aluminum heads and lower intake from edelbrock and a fairly mild cam.
The parts you are talking about buying are what Grady was talking about.

Then you'd need headers to take advantage of the heads. Then a new carb. Then, you may as well rebuild the engine. (I won't mention the "may as well, while I'm at it" that will befall you when you get into engine parts. "Le Mans" rods cost how much, now!!!???) Then you'll want better brakes. May as well put some aftermarket wheels on it, too. Then maybe a shift kit for the trans. Some gears for the rear end. On and on.
Ask me how I know. I had a 1966 Fairlane. (I LOVE that body style!!!) It was a fun little 2v 289 auto car. I swapped up to a 351w (Windsor Sr's, Hooker Super Comp headers, Holley 750, Performer RPM intake, Mallory ignition stuff, etc. It WAS a BEAST!!!!) and Toploader 4-speed. Then a 9" with 3:55 gears. Then disc brakes from a Torino. On and on.
I sold the car for a loss, because I couldn't afford to go on adding stuff. The floor pans were starting to "show through" and the rear springs started sagging, etc.
That's a lot of money and it's coming from a college fund. I'm really not trying to talk you out of buying it. (Not my place. Just adding my opinion to this mix. I did the same thing when you were talking about the Poncho, a few months ago. Old cars typically cost at least DOUBLE what you expect to pay. It's just a fact.) It's just that once the car is in the driveway, the allure and romance start going away. Then all the little issues start becoming like the proverbial "small rock in your shoe". Sorry if I seem negative. I'm not. I've just been down this road, myself.