You do have to loosen/remove the main caps to replace the seal correct? If the mechanic that did the seal re-torqued the caps incorrectly that may be the culprit.
Would be my guess without seeing it first hand....
What does that end up causing?
1968 Firebird 400 Convertible Original: Engine, Intake manifold, Heads, Body, Stereo, 3.08. THM400. Engine rebuilt by Ken Keefer early 2012. New: Holley 4bbl 670 HP, hidden iPod setup, RobbMC mini starter, A/C, front disc brakes