It should work, but as mentioned before, it is always better to amplify a stable oscillator signal than generate all the power in one stage.
Note that the PL504 does not have a standard base size, it is bigger than most tubes. Also note that the heater alone needs 10W of power! It is designed for 0.3A series connection but needs 27V across it.
I think if you are trying to produce more than a few Watts RF power it would be wise to use a large quartz crystal. I suspect if you use a very small one it may be overloaded and have a short life. Be careful about he voltage across the key contacts, it may be as much as 40V with the key open.
Brian.