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34 watt T12 tubes aren't compatible with T8 ballasts. While they *may* start, they won't be operating efficiently and could burn out the ballast. They aren't even a particularly good match for many T12 ballasts.
T8 ballasts are made in sizes that vary from 1 to 6 T8 lamps.
Perhaps better yet, remove the ballast altogether and replace those tubes with LED tubes, which wire directly to the 120VAC line. It's fairly easy to do, and for 20W per tube, I get MUCH more light in my old shoplights.
While I had them dis-assembled, I cleaned them (overdue anyway!) including wire-brushing rust spots, and painted them (especially the reflector) gloss white. ZAM!