Looking for powertrain input for a project

Hi all, I am not a trucker, and here is my story...

I have a 1964 International R-195 (It was all lonely on the side of the road so I brought it home) That I am going to completly repower and while I have learned a lot, I have come here for advice. It's current setup is a Red Diamond 450 pounding out a whopping 166hp and 380ish tq, 5 speed manual OD trans, and 2 speed rear with ratios somewhere around 10 and 6 to 1. It tops out in the mid 50's and gets 6mpg. While this may be good for a resto purist it will not cut the mustard for me. I am planing on using a hopped up 8.3 cummins for an engine since it is almost exactly the same size as the RD450 and I just like cummins lol. The part that is still up in the air is what comes next. My idea was to use an eaton TX14607b / spicer 8341c / 2.5 ton military t-case with 1:1 hi 2:1 low (I already have this) / 5 ton rockwell axles 6.44:1 ratio / 16r20 Michelin xzl's (already have these too). With both cases in OD it would have a combined ratio of .63 and allow me 65mph at about 1650 rpms but I worry that I would have a LONG front driveshaft. Will the driveshaft be an issue? should I look for an axle with a ratio closer to 5.40:1 and scratch the brownie? I really dont want to, but should I look for a double OD 13 speed or higher trans with .62 final? Most of the time I would be driving this empty on the road, but I want it to be able to drive over anything, or pull anything that I hook it to with out needing a CDL. Oh yes, the truck is 26 feet long, and will be going with a single rear axle when finished. Look foreward to the ideas!

PS, will post a pic as soon as I am allowed lol