Hello. Several possible solutions popped into my head reading your question, but as I write this I'm not 100% which I would choose. So here they are for you to ponder and consider...
First, you could use the OT HRs field to record those hours above the 40 hrs (capped), making your overtime rate as 1 instead of the typical 1.5 or even 2.0. Then in reporting you can the two together to get your actual (or create a third field with
a formula to autosum them).
Second, you could keep the actual work (aka hours) as is and simply put in a custom field for the capped hours for cost and reporting.
Third, flip idea #2, and make actual work your capped hours and use a custom field for tracking the "real" actual hours.
Hope this gives you some ideas of possible things you could try.
Maybe post back to let us know what you decide and why. Or to give more information.
Good luck!
Christine