1968 Barracuda - Let it begin (the last build!)

If it was me;
I'd do all the mechanicals first, and then drive it for a few months to iron out the kinks. Like you know; overheating, failed electrics, lousy alignments, bigger tires two or three times, and finally back-half/tubbing it, etc. There is a chance that the engine will be in and out a few times before you get it all fitted. And the trans or convertor is almost guaranteed to not be exactly what you want, so it may have to come down a time or two. Depending on so many things, the first chosen rear gears may be wrong. The rear end may have to be narrowed,etc. The list goes on and on.
In all your testing, you may crash the car! Or someone might crash into you!
So IMO,
I would do all the mechanicals one winter,
then spend the following summer dialing it in.
Then do the bodywork and any welding the next winter.
Then whatever mods you didn't get finished, during the second summer. Like spoilers,scoops, fresh-air ducting, and such.
Then the third winter, after you got it all dialed in, then blow it all apart for paint and a new windshield, and spend the rest of the winter carefully putting it all back together.
That's about how I'd tackle it. This way your waaaaaaay pricey paintjob has the best chance to survive.
PS
I was ~44 when the bug bit, but because of both budget and time restraints,lol, the car didn't hit the road until about 2.5/3 years later.. I then spent about 4 summers ironing out the kinks.

Happy HotRodding,