If the power supply is giving 425V now, it is fairly easy to reduce it to 360V-380V:
Just change the first 250uF after the rectifier to 470nF 1000V FKP/MKP (LCR components type PC/HV/S is recommended for this position see Farnell).
PC/HV/S/WF 470NF 1KV - LCR COMPONENTS - CAPACITOR, 470NF, 1000V | Farnell United Kingdom
The value of the 470nF can be varied from 220nF to 1.5uF to adjust the voltage output, once you have the amp running.
This change makes the power supply into a (quasi) "choke-input" supply (you can look this up). If your choke is a Tribute 10H/200mA, it will work perfectly in this duty, provided the 300Bs are biassed to keep the total supply current below the rated value.
Choke-input supplies have many advantages: the "conduction-angle" of the rectifier is much greater, which reduces the rms current (and therefore the stress) in the transformer and rectifier. The regulation (change in output voltage vs. current) is also improved. More subtly, the reduction in rectifier peak current means that there are less electromagnetic emissions from the transformer and wiring (which may get picked up by your signal circuits).
With such high-quality transformers, your amp deserves the improvement in the sound that choke-input should give you.
Another way to reduce some voltage: Change the rectifier to type 5R4GY or (BRIMAR or RCA) or Chatham 5R4WGY. These type drop more voltage than 5U4, and also sound noticeably better than 5U4. In this case, the first capacitor must be 4uF or less - but again, choke-input will be better still.