I am swapping to a 454/TH400 in my 70 Chevelle.
It currently has a 350/TH350.
Will I need a BIG BLOCK wiring harness?

you should be able to use your 350 harness, might have to add a little to a couple of them, like the oil pressure sender.

What kind of shape is it in?

When I converted my SB to BB, the wire harness was so brittle that I didn't want to take a chance so I purchase a new BB wire harness from M & H, that way I shouldn't have issues down the road.

Just my 2 cents.