Indenting lines 400..457 help

[Ned Harvey]

For the life of me, I can't find any easy way to indent (or unindent) lines
numbered 400..457 or whatever.  Does anyone have any good suggestions?

I'm usually a vi user, but I'll happily learn a command of some other
format.  I've tried
    :400,457s/(.*)/   \1/g
and a ton of variations of the same idea, but it seems that the
search/replace doesn't do what I expect it to.

Thanks for your help.