Lots of good advice above but ill chime in on drivetrain question by asking what is your vision of final product?
On front end first decision is i beam or IFS. If I beam you can upgrade brakes to disc and put in dual master cylinder to get better stopping ability . Better shocks and check condition of leaf springs. If IFS ther are many "Mustang" aftermarket kits or research stovebolt on narrowing a 80's bolt in. I put a 69 Nova subframe clip in my 49 burb which Is another option.

On engine/transmission upgrade you can upgrade to later model inline or swap in anything from small v8 I.e 283 to late model LS. Tranny can be from T5 behind inline to full electronic overdrive 4L80 versions. Just keep in mind the more "exotic" you go the more fabrication needed to fit 10LBs of stuff in a 5LB engine compartment. I have a 80's vintage 454 BB so again depends on your vision.

On rearend easiest is to make sure leaf spring, shackles, and shocks are in good shape and find newer rearend with better ratios. Also several kits to go to full bore triangulated 3 or link . I used same 69 nova rearend with offset rims to get correct stance inside the fenders.

Sorry for long diatribe but just say options are almost limitless depending on your end goal. Create that first and do what makes you happy. There are great restorations on Stovebolt from craftsman I truly admire but I don't have patience or ability to go that route. Just keep it fun, get it on the road and enjoy the journey.
P



Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
Things may come to those who wait...but only the things left by those who hustle."-Abraham Lincoln