Big engines could have big torque converters. You can easily modify the differential ratio in the suspension_r file by changing "ratio"
I would need to make new differential for automatic really. What I have found so far is that greatest stall ratio has been 3.98 in any torque converter data I have seen, so 4 is not that much more really, BeamNG Wiki has not so high values listed, but maybe it works still, however higher than ~2.7 have usually been found on stationary applications. Allison TC 950 had Stall ratio of 3.67, that is highest ratio on Allison I have seen (of course there can be higher!), max engine speed for that is 2100rpm and max power 600hp. It weights 370kg and housing is 711mm in diameter, torque converter inside the housing is of course smaller, but dimension for that is not given, but by estimating and calculating pixels, I ended up with a 438mm, which can be far off. This is best document I could find on those and I'm not entirely sure if those are for truck use, they might be for stationary use for winch etc and those tend to have greater stall ratios: http://www.copelandintl.com/sites/default/files/TC Specs.pdf I might try with that data, but it might not work out too well, anything below 0.48 seems to be too little with that loaded truck. So I guess even more research is needed, I really would like to get realworld data in for starting point. Differential ratio of one truck that uses Allison 4000 transmission was 4.1, so changing differential ratio to that might be a good idea, some other truck had 4.88 and semi in BeamNG has 3.0. --- Post updated --- There is two issues I don't get yet, it does not want to go in reverse too well, even if I lift rear end up into air, looks like that driveline is freaking out, not sure if transmission, TC and differential reduction becomes too great for driveline to handle or if I have made some bug by myself. Second issue is that sometimes, very briefly it shifts to 3rd from 2nd, when it should shift to 1st, this could be cured by increasing torque converter size, but I think it is already at maximum for T75 with T350 turbo. Put T400 turbo and issue becomes worse. However it drives nicely and only reversing issue is really bothering one as that second issue is quite rarely happening for me at least. There are now differentials too, Automatic locking differentials, that have shorter ratio. Do what you like with it, I don't really care as long as you have fun.
Few tips. If reversing is difficult, reducing diameter of torque converter can help. If box shifts up instead of down (at full throttle, speed decreasing in a hill), it can be that you need to lower shift down rpm of bigger gear, or/and increase shift up rpm of lower gear. However with 1st and 2nd gear torque converter stall rpm has to be kept in mind too, I'm not sure, but certainly felt like there would be only 100rpm window where it can work, with my current ratios, I could not put shift points where I wanted them, it could work only by very limited rpm range. Now just needs adjustment of overall feel of the gear changing as shifting works now properly.
