If fuel mileage is a factor, why not stick with the LN3? The LN3 and L27, especially when paired with the 2.84 FDR are highway cruisers, which, when modified and tuned slightly, can get crazy fuel mileage, and the fuel mileage is not easily affected by speed.
Admittedly, the above mentioned FDR paired with the LN3 or L27 does require some encouragement to get moving, but that is what the mods are for! The LN3 does love the 2.93 FDR.
BTW, you can plug the VIN into compnine and get the FDR from that, or check the RPO sticker on the spare tire cover.