I'd suggest taking it one step at a time.
You have mutually exclusive objectives, and you'll have to sort those out, but by starting stock, where you are today, and only changing what you must to keep it going and making only slight improvements you might find a compromise that works for you.
Changing the ring and pinion to the 3.55 will gain you some speed, but you still won't run 70. To get a better ratio you'll need to swap the rear and that's a more serious change.
If you have to do serious motor work look for a 235 or 261. You'll gain pressure oil and some abilty to spin the motor faster reliably. Most people won't be able to tell the difference.
I don't think you can get an overdrive in place without changing to open drive, and at that point your changes are far more serious.
The disk brake conversions are bolt-on things, and you can improve stopping that way, while still being able to go back stock if you ever feel the need.
Change just a little at a time. You'll find the middle ground between stock and drivable that will serve you.
Good luck!
