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As a means of protecting the IRF510's, I think a polyswitch in the PA-PWR line should work great.
The IRF510's mostly fail due to overheating, a small polyswitch will have far less thermal mass
then the IRF510's plus heatsinks.

Often an LED and resistor in series are placed across the polyswitch,
such that when the polyswitch opens up the LED turns on.
That tells the operator to shut down power and let the polyswitch reset.

Would be interesting to experiment a bit, the IRF510's are cheap.
Turn up the gate bias pot till something gives.
My bets are on the polyswitch (rated for say 2 or 3A, will blow somewhat north of that).

I doubt you need a limiting resistor in this particular case.?

Jerry