You're probably good to go with a 15 minute break-in. As long as there's enough oil splash from running well above idle, the cam and lifters establish a running fit pretty quickly. The whole reason for not idling after startup with a new cam is to flood the cam/lifter junction with plenty of splashed oil off the crank and rods to prevent scuffing while the new parts are getting acquainted with each other. A stovebolt doesn't have the extreme valve spring pressure that some high performance engines do, so a short break-in run doesn't present any real concerns.
Jerry

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