Yes, it's a simple swap. 1N5408 diodes should work fine unless your amp runs some crazy 1000V B+ with transmitting tubes or something.
I don't know where you heard 20 watts per fin, but it's almost certainly wrong. If anything, it's 50 volts per fin or something like that.
Selenium rectifiers should be replaced whether they're working or not. If they ever decide to blow out, they'll stink your house out with evil-smelling toxic fumes.