There are two ways of protecting a transformer, primary only and primary and secondary. If you use primary and secondary protection, your primary OCPD can be 250% of the primary current, in this case, 273 x 2,5 = 682 amps, so your 400 amp breaker is fine. The secondary full load current is 630 amp x 125% = 787 amps max ocpd. Your proposed total of ocpds for the secondary side is only 625 amps, well within the limits. you are allowed multiple ocpds, see note 2 to table 450.3(B)
Conductor protection is a separate issue from transformer protection, so parallel 3/0 conductors on a 400 amp breaker is a good pick.
Edited by Tom (12/15/12 06:28 PM)
Edit Reason: added conductor comment
Few things are harder to put up with than the annoyance of a good example.