We're in two different worlds. You're tire size is massive and you're running a diesel that doesn't like to rev as much as the hemi. If it was me, first thought would be shooting for close to stock revs or maybe slightly higher in 6th gear. With my 35's and 4.56's I'm revving around 2k RPMs at 70k, which for me is perfectly acceptable on a hemi.
I like this calculator personally:
http://www.grimmjeeper.com/gears.html
I believe you're year started using 3.73's over the previous gen's max of 3.21? If so, then 4.88's puts you right back into stock RPMs at 70mph. That would be my choice.
I believe the 68RFE's weak point is the overdrive clutches and excessive loading. Probably a safe bet that those 43's would be excessive loading. Even if you're not towing. I have no first hand experience, but I've read posts of guys wearing out the overdrive quicker than expected on 37's and having elevated power levels (i.e. deleted trucks). I have no feel for how often that happens, just regurgitating forum posts.
There's a youtube channel, Beast Project or something like that. He had a 6.7 on 37's and went with 4.56's. He said something along the lines of regretting going that low in gears due to drop in highway mileage. He also recommended regearing asap if you go larger tires as he experienced trans issues. Look him up, search beast and 4.56's and you should find his channel.