don't ya just love the dreaded 3rd-4th hole? .. With the stock tranny/xfer, you might know you already have a few splits in there to break up the 3rd-4th Hi hole, they being within the sequence of 2nd Lo, 3rd Lo, 4th Lo, 5th Lo, 4th Hi , 5th Hi and vice versa for downshift with a double-clutch into Lo side 5th after you start to lose speed in 4th Hi.. Gotta catch it early. It is not a perfect solution for the dreaded hole, but I have been using it since my first days of being shown it in army MTOC... Granted, I still curse the one(s) responsible for deciding the ratios in the 6453 spicer would be okay in a straight-stick truck, especially any 6x6 that's a bit underpowered for the road to begin with. I have also experienced the 6453 (and 6352-direct) in a few 1970s civilian trucks. Their 2-speed rear axle ratios made the dreaded 3rd-4th hole not nearly as severe as it is in 5-tons... IMHO, because of the Lo side available in a 6x6 transfer case, there appears to be no logical reason as to why the dreaded hole couldn't have been eliminated by using a tranny having a faster 1st, 2nd, and 3rd gear ratio.. Every time I am heavy and pulling a hill, I still curse just like I have done for 45 years ..I mainly want the auxiliary box so I can have two more gears (3 over, 4 under) between 3rd and 4th.
The spicer 6352 direct-5th has better ratio spacing than the 6453-OD and not as severe of a 3-4 hole, but it cannot be put in a diesel or multifuel without losing up to 10mph top speed, depending whether it's put with a mack, cummins, or mutifuel.. Yes there are some A2 rebuilds that have the 6352 direct, my M62 is one of them and I am happy it does because it needs all the help it can get to get itself down the road, lol
Don't get me wrong, both models are good trannies, but there must've been another good spicer model better-suited for our favorite trucks, instead of a couple that were deemed "not perfect but close enough"