I doubt you could use them UL and get decent power.
The max rated screen voltage is 250v, and even with
a 250v plate supply, a UL amp would exceed that on the
screen on every positive half-cycle.
Unlike EL34s or EL84s, they weren't
intended for ultralinear circuits.

Gary always ran the screens from fixed DC, and
even though he was running 330v, the tubes
were stood off ground for direct-coupling,
so the screens were always within rated operation.