True, simplicity is a great thing off road. But if I were starting from scratch, I would do a trans with overdrive. Since I am not a trans guru and have not spent any time researching it, I don't know which I would do, or which is compatible. So, I have no advice for that.
Since you have options at the moment, I would go with the overdrive.
GLFWDA member since 1979.
Member Southern Michigan Rock Crawlers.