Howdy,
If you look a little closer at the tubes, they should say "Six Jay GEE Six", not "Six Jay SEE Six".
With those tubes, 70 to 100 Watts carrier, 350 or so peaks would be reasonable. Might show less. Tube testers don't put more than a tiny trickle of current through a tube. Doesn't push the tube to it's current limit. Makes it hard to tell where that limit will fall.
Types 6JR6 and 6JT6 are about the same size. The 6KV6A is larger, and tougher to boot. And rare. Might see more power with that one.Not much, but more.
If you don't put some fixed bias on the grid pin #2, they may overheat with the carrier turned too low. Or too high. As a rule, you'll find pin 2 soldered directly to ground.
And one of those thirty plus-year old filter caps may go "SNAP" and plaster its guts all over the underside of the chassis.
73