From our research the big advantage would be the ratio options to help offset the 40" tire. We currently do not run an underdrive in our truck because we ran outta money and time towards completion of the truck haha. I did notice we were lacking in some places around the track power wise buuut we have a few other issues to sort out with tuning before we blame our shortcomings on the lack of an underdrive.
What a 5.43 didnt cut it with a 40 We could have told you that. haha For referance GHA racing runs a 5.83 with a 39" and loves it. Also doesn't feel the need for an unerdrive. Hits over 120mph. Try a 5.83.