Hello,
it's not useful to ask about 6.5, 7.5 or 8.5 digit DMM calibration.
You only need to compare the possible uncertainty of the 544x A/B vs. the uncertainty of your intended DMM.
In principle, as the 10V output of the calibrator is extremely stable over time, much better than specified, and maybe better than usual 10V standards, it's also possible to calibrate DCV of a 3458A.
I append the specification table from a Fluke / Philips catalogue.
I have the 5440B_AF operation and service manuals, and both contain the specifications, of course. There are only small differences between 5440A / B, and 5442A as well. These might be available at xdevs.com.
Frank