I suggest that you email or phone the people at PartsConnexion.

They have almost every type of capacitor and other components you can Imagine.

They have caps that are specifically recommended for guitar amps and those that
are for high-end preamp coupling caps.

Call them and talk to someone there and you will get some direction and recommendations.

They also have DIY tips and plans.

From the wiring diagram, the twin 6L6GC output tubes may be a nominal 40 W output. In these tube circuits, the coupling caps have a high voltage rating, and are usually a small capacitance value on the order of 0.01 mfd. Just be sure you get a high enough voltage rating on the caps.