I would rip all the front-end tubes out and drive it with a 10W TOA amplifier. You can get these cheap on ebay and other websites.? $20 to $30 bucks. They have 4/8 and a 70v output with balanced and unbalanced and a Mic input. These work great to drive a pair of modulators.? They also have bass and treble.? I use several here for that purpose.

The 813s as Chuck said, wont play well on higher bands. I would not use those. A 4-400 single is the way to go here.? You can modulate the screen with a Dropping resistor off the plate.? No screen supply.? ? You could use the 813s as modulators though!

Technically you won't even need a bias supply for the final. If you build a simple sequencer with a few relays, Diodes between the coils and caps to set delay.? You can ensure Drive appears on the 4-400 before its keyed with B plus.? Set drive to -183 volts using a meter and a choke right to the grid.? This will be 10 to 12 MA.??

You could Grid leak it, OR build an adjustable bias supply with negative -75 to - 175 adjustability. I would not bother. One of my rigs has been running 20 years with No Bias.? It has a simple over current relay.? If drive where to disappear, it just trips.

A single will truck along at 500 to 600 W carrier No problem.

C