So, to answer your question, since no one else did:
How to replace the selenium rectifier: (Read all steps first.)
1. Locate the 2 wires from the transformer. I am sure that these are the blue wires on top, but just check by tracing them back to the transformer. The 2 blue wires are AC, and thus interchangeable.
2. Cut or de-solder the blue wires from the old rectifier. Solder them to the 2 terminals on the new bridge rectifier that are marked "AC" or marked with a tiny sine wave. Again, It does not mater which is which on the AC side. If you have to add wire, keep them twisted as close to the new component as reasonably possible.
3. Determine which of the 2 wires going down into the chassis is connected directly to the chassis. De-solder or cut from the old part. This may seem counter-intuitive, but the one connected to the chassis is connected to the terminal or lead marked (+) on the new bridge rectifier. This is because this whole circuit is a (-) bias supply. Point (G) on the schematic is at a voltage below ground. You'll also note that all of the capacitors in that circuit have their (+) connected to the chassis. Again, this is correct, as the voltage they are filtering is actually negative compared to ground.
4. Verify that the remaining wire connects to the same place as the 2 resistors (R30 and R31). De-solder or cut that wire from the old selenium rectifier. Solder to the new rectifier (-) terminal or pin.
5. (You may actually want to do this step earlier in the sequence). You have to mount the new component. It can be mounted directly to the chassis; bridge rectifiers almost never need to be insulated from the chassis. Mounting it underneath would probably be better. A little heat sink compound would be appropriate, but assuming you bought the correct part, it does not need airflow like the old selenium rectifier did. The side, near the rest of the bias circuit would be good, as long as a screw head on the outside will not interfere with the case.
6. (And this is where it gets a little complicated.) The new rectifier will have a different voltage drop than the old part. This will negatively (pun intended) impact the bias voltage. The lower (higher in absolute value) bias voltage will make the amp run "hot". It may --> will stress the 7189's if you do not check/set the bias.
You will need to replace R30 with a slightly larger value, or add a small value in series. Other things like component degradation and household voltage affect this as well. See the dozens of posts here about biasing. Most people install a variable resistor to allow it to be adjusted.
According to a spec sheet I looked at *with very little research*, you want 65ma cathode current with the amp idle (inputs and speakers connected, but no music playing). You can measure the voltage across R35/R135, and do some maths, or use a bias meter. As a *pure guess* I'd try changing R30 to 12K (use appropriate wattage part.) Again, lots of articles here in the AK tube forums about biasing.
