Thanks for the update and extra information!
Typically, 60' hops (bittering addition) have given most of their bitterness after 30-40' in the boil. So even in a 60' boil their IBU contribution will not increase all that much from the level achieved at 30' or 40'.
You may also enjoy higher bittering efficiency during the boil than the recipe anticipates.
Are you boiling full volume? When are you adding the extracts? What kind of water do you use?
But... flavor and aroma hops (late boil additions) will continue to isomerize as long as the wort remains above 160F. So chilling fast becomes crucial to prevent that extra bitterness from developing.
A good alternative to adding those late hop additions at the end of the boil is to add them as "whirlpool hops" (aka "hop stand" or "hop steep"). This addition is made when the wort has been chilled to 170-180F or perhaps even better, at 160F. The bittering reactions are far less at those reduced temps. A 10-15 minute hop stand with a good stir every 5 minutes, will extract flavor and aroma, while not adding much bitterness.
How long does it take you to chill from the boil to 160F? During that time bitterness will still increase.
Could you post or link to the recipe and brewing instructions?