I have same brake cable lines on the down tube , and just slipped u-bolt between cables and frame. also added a piece of rubber tube over u- bolt before installing to keep from scratching frame too bad , I dont have a problem as far as having to stretch cables around u-bolt , cables do touch u-bolt , but not bad ( not stretched). as for your down tube , it wont hurt to try first before you have to drill through frame!

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