Here's a couple of quick tips I use when making epic-level critters that might be helpful:
Monsters have 7 non-epic feats at 20 HD.
To determine the number of epic-level feats, use the following formula:
([Monster's HD - 21] divided by 3) +1.
Thus a 30 HD monster would have ([30-21]/3) +1, or 4 epic-level feats. (One for 21st, 24th, 27th, and 30th HD).
Once a monster's total HD exceed 20, it may select either an epic feat or a nonepic feat for any feats it gains at higher HD.
Save progression stops at 20 HD. After that, epic save progression kicks in. Thus monsters will have +12 in their good saves and +6 in their other saves at 20 HD. To determine the epic save progression, use the following formula:
(Monster's HD - 20/2)
Thus, a 30 HD monster would have (30-20/2), or +5 epic save bonus. This bonus is added to all saves, regardless of whether they were "good" for that monster type. Thus a 30 HD outsider would have Fort +17, Ref +17, and Will +17 when adding its epic save bonus to its nonepic base save bonus.
Base attack bonus continues to advance as it does at nonepic HD.
If I'm wrong on any of this, please let me know.