I'm surprised nobody's mentioned being sure to have (2) hot leads and
neutral, so you can power 117v circuits.

I've done this sort of thing many times over the years, but via single
117v 15a or 20a branch circuit. Main opened first, ditto all breakers
(which are accurately labelled as to which load). Of course circuit is
chosen so as to power selected "phase" which luckily has covered the
necessary loads. Then one breaker at a time on, and maybe rotated as
necessary. (Refrigerator does not need power 100% of time, nor does
furnace blower. Yada, yada.)

At a friend's house, opening main breaker one time, caused it to fail
to close forever more. Which sped up replacement.

HTH,
J