I would turn it into a GTX before a road runner, 440 is a must (400-500hp) with an FI tech system, leave it an auto, find a used dana, pst suspension kit, some 15" rallyes with new slippers, front disc brakes, nice exhaust system, go through the steering, new shocks, and some new t bars for the front to hold that new 440, a classic a/c system (if it doesnt have ac), replace all the light bulbs and flashers with led's, throw in a nice aluminum radiator, maybe a mini starter, new battery, new belts, hoses, heater core, and a real nice stereo system..
Then drive the wheels off it...
but thats me, you really have to just start making a list of what you expect out of it and then see what you have to do to get it where you want it...
I have been in your position, I had a 70 coronet 500 (383 auto), so clean and original, it was a 2 owner car, in great shape, always garage kept also, super super solid..
So what do I do, keep it the "no fun" 500 "grocery getter"? Since i got in May of that year I decided to drive it that year and take some time to make my choice..
After driving it and taking it to a few shows (the paint was amazing, burnt orange in and out), I decided I had to clone it into an RT, it didnt grab a bunch of attention the 383 was OK, but a stock 383 with just under 100k on it, leaked oil, had a bit of blow by, the 727 was lazy, there was no posi and highway gears...
So I made a list.
R/T appearance package (side scoops, n96 hood scoops, 15" rally wheels, and badges)
Disc brakes up front , tight suspension, performance steering, and performance tires
440 and rebuilt trans with shift kit and fancy shifter (it was a long time ago, I admit I cut the column shift off with a file and installed a bm on the floor, I am ashamed. (450hp) (back then fi wasnt easy or cheap, no fi tech is so cheap for a driver its not worth messing with a carb, we still drive the cars when its cold, so carbs suck in the cold, about 3 weeks ago it was 8 degrees out and I took the gtx to work, fi tech fired it right up no pumping the gas, no crossing my fingers, lol... granted some carbs can run great in the cold, its just not in the cards for me to have one, lol
A/C, is had it factory but long since worked
Tach (non rallye dash, back then that little tach was going for $500-750 )
rear end I wanted posi with mid 3 gears, ended up finding a 354 dana at stafford springs show for $550, score back then, and now would still be a great buy.
Anyway that first winter i did it all, I bought the car for $8500, and spent all of that again doing the work, the good news was didnt need real paint work and the interior short of the carpet and headliner were perfect, I also changed the windshield, cant stand old glass in the front...
Keep in mind the car was probably worth $15K at the time, lol. Luckily, I kept it long enough to sell it for well over double my investment a while later...
I drove that car the next summer and night and day enjoyment out of it, now it was a fire breathing RT, and it got more attention, got worse mpg, and wasted a lot more tire, but it was fun..