i had actually planned on keeping the four spd tranny in the truck so i could find a 2pd unit to bolt behind it, i figured this would be easier than installing the allison , plus trying to figure out how to get a 2spd unit behind it might be quite a challenge, besides selling the ally could offset some of the cost. ive driven bunches of 6.2,6.9 trucks with 4.10 rears and no od. talk about reving, like you say a 5.9 would really be a slow boat plus wear out quicker.