Mark,
For the 379, the cab moves rearward, as do the battery/tool boxes. You will need to lengthen the exhaust piping and air cleaner piping.
The cab also sits lower on 379-127" compared to the 378. The cab mounts are lower all around.
For the 388, the cab will be further back than 378 (119" versus 123" for 388) and the cab will sit slightly higher than 378.
For 389, (which has a 131" BBC) the cab will be slightly higher than 379-127. Still, much lower than 378.
If you want to keep all the kit frame mounting holes in the right locations, the best place to add length to the rail is ahead of the battery boxes. For 388 you'd be adding 4 inches. For 379-127 you'd be adding 8 inches and for 389 you'd be adding 12 inches (in 1/24th) to keep all the other components lined up with the cab.
If you lengthen the rails, you will need to lengthen the driveshaft.
Tim