the best place to hook up the power wire on a new MSD box on a 1970 challenger is??

If your talking about the 12V on and off wire take it to the fuse box that will provide 12V in the on position, that or splice it into the ballast resistor
ignition feed wires, both in the start and run positions

I ran a hot feed from the big post on the starter relay to a new auxiliary fuse box (actually I have 2 of them) and I run all of my accessory stuff on it. Fuel and water pumps, ignition box and gauge lights all run of of one and the other is 12v keyed power for electric fans and dakota digital controller.

Per my MSD instructions, I powered it directly from the battery.

if i remember correctly.
if you use the ballast wiring for the key on power you will need to splice the brown and blue wires together ( i think that is the power wire colors ). one has voltage at key on and no voltage at key start. and the other has no voltage on key on and voltage at key start.

I did the same thing.

Their wiring instructions are easy to find on the 'net.
The 10ga wires to battery is best, starter relay battery post works.
Main thing is the box never sees interruption of power/ground/signal when running.