Most NAPA stores have equipment to fabricate high pressure hydraulic lines. I'd suggest taking the master and slave cylinders to one of those places and see if they can custom-make a line for you. The only place a flex line would be needed would be a short one from the firewall to the bellhousing to absorb flex from the motor mounts. A hard line will fatigue and break in short order.

It's possible a flex hose from a front brake could be adapted to that application if the fitting at the slave cylinder could be matched up. The frame lines will usually have 3/16" tubing inverted flare fittings and pre-made 3/16" steel lines are available in all sorts of lengths.
Jerry

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