Going from direct (1:1) to O/D (. 73-. 75) is still a bit of a jump on a Eaton. The O/D ratio would be the same, or very close to what you already have in your 5spd. You would want to find a transmission with a double O/D, like a Mack T2130. 12th is something like . 80, and 13th is around . 75. They are heavier than a Eaton, though. And are a triple counter-shaft transmission, IIRC.
A Gear Vendors or a US Gear unit would probably be easier and cheaper to buy/install.