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I would consider 5uF to be undersized in a HV tube amp power supply filter. Either your Bogen has been upgraded to 20uF, or the design changed, but the schematic didn't. Either way, it is an improvement. If you want to use the original 4 slot mounting insulator to mount the new can, you could get a 20/20/20/20@450v can, and jumper two of the 20's to make a 40uF for the first node for a little better filtering.
Mfd is the same as uF, both express microfarads. In dealing with caps, especially in high voltage situations, the tolerances for capacitance or uF is pretty loose, and not highly critical, so the difference between 20 and 40 uF isn't that much. However, the voltage rating is more important. Always use a rating at or higher than designed. For instance, this four cap 20uF can is rated at 475v, where the original is 450v, making the replacement an improvement, especially with today's higher wall voltages. https://www.tubesandmore.com/product...f-electrolytic
And if you mess up your first amp, I know a guy....