Nice, and congratulations! Please do a Tear-Down and a characterization, please.
The black 3458A and the 5720A seem to agree within 0.01ppm.. even nicer.
Now please check DCV impedance, bias, noise, INL of DAC. If you can, noise spikes from the multiplexer, in comparison to the old instruments.
Check stability of DCI in terms of T.C. and time and consistency against 5720A artifact calibration.
FW should be 8 or 9, as it didn't change. U180 also did not change.
From pictures of the Repair Manual, take notice, that all the logic and the 68000 is included inside one gate array, apart from SRAM and Flash firmware.
On the A/D board, logic and µP obviously also was crunched into another GA.. how many layers does this PCB have, in comparison?
Btw.: An original paper version of hpj 4/1989 is still down in my basement lab... together with the complete original manuals
Frank