Lots of choices... though I think there are a few things that apply to all your goals. FWIW I also started with a 1970 318/auto Challenger.

First is brakes. 4 wheel drums are just not enough when the guy or gal in front of you slams on the disc brakes. You at least need front discs. There are lots of ways to get there. There are more and better options if you upgrade to later E/later A "big bearing" spindles. I would start there. Then pick out a front disc option. Dr Diff has quite a few. The factory style 11.75" setup is good and many options beyond that now or later. 11.75" discs will need at least a factory 15" wheel. Smaller discs available will fit a 14" wheel... but this is a performance application so I would suggest you plan on 15" wheels minimum.

Next is the backbone of the chassis. Whether straight line or cornering the unibody can use some help in stiffness as performance level is increased. Frame connectors are high on the list. There are many options... as long as they are weld in they will help. I used Auto Rust Technicians 2" x 2" square tube which fit below the floor and they help a lot.

All of the above plus suspension rebuild and maybe a bigger front sway bar will make a fun street car that won't fall down on the track. Firm Feel helped me on the suspension side.

Beyond that, it starts to matter if straight line or handling is the goal.

My '70 is modified for handling and decent road manners.