Second paragraph under Increasing Hit Dice on page 291 of the 3.5MM-check against Table 3-1 on page 22 of the PHB.
Monsters do not go epic with regards to saves and base attack bonuses-just extend the formulas.
Monsters do go epic with regards to epic feats, but I've little clue where. As a rule of thumb, assign monsters epic feats when they can take them(when they qualify to pick feats from HD of 21 or above) _and_ the encounter EL is 21 or above. That's worked well enough for me.