in a 70 without removing the fender or brake booster? It doesn't look possible...

Is it possible to replace the ignition wire in a 70 without removing the fender or brake booster? It doesn't look possible...
Yes it is. You can remove the firewall bulkhead, remove and replace the wire completely (you need Packard 56 series terminals to do this properly) or just cut the wire a couple or inches from the firewall bulkhead and splice a new wire from there. The couple of inches of resister wire will be fine for most apps, depending on what you are trying to do.

OK. I'll give it a shot....

If you drop the inner fender well it's a little easier to reach.

You can also remove the two screws that fasten the fuse block/bulkhead connector and pull the assembly inside the car. The single screw that holds the forward lamp/engine harnesses can easily be removed with a small (1/4" drive) socket. Now separating them is another story. Sometimes they're easy sometimes hard, if it's the original connection it will likely be hard.