Instead of trying to put 1 light bright enough to light the whole truck, I put several in Frank's '75, each with a pair of aimed lights (for each occupant) and a dome bulb (on the door switches).
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There was also a cargo area lamp at the t/g with its own switch from door-triggered to always-on:
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They use smaller, cooler bulbs, but lit the truck better than 1 bright fixture would have. And if he still had the truck, we'd probably swap them all to LED.