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What motivated the choice of the IRFP4668, vs the higher power and lower on resistance of the IXFX230N20T? The lower gate charge?
Have you considered using the a dedicated SR driver chip such as the IR11672AS or ZXGD3101T8CT? They sense the gate-drain voltage to emulate a diode, even tracking the PWM frequency to anticipate the transition. There are many similar chips out there, but the two above have a good combination of few external components, handling the full voltage without a voltage divider or input clamp, and having strong gate drivers. I haven't found ones in a through-hole package -- these are at least big enough to hand solder.
The gate drives for SR controllers might initially seem a little weak, but remember that the Miller charge is actually helping rather than hurting. And the intrinsic/body diode is still working, even if the gate is slow to turn on. The only performance critical region is the turn-off: an early turn-off lets the full body diode capacitance build, while a late turn-off allows shoot-through current. A perfect turn-off has the gate region narrow with a short carrier lifetime.
The neat thing is that you can add this circuit with only connections to SR MOSFET pins and a "bootstrap" diode connection to the low-side gate driver power supply. Note that you want the power supply V- to come from close to the low-side MOSFET drain, and SR MOSFET 'source sense' connected to the high side source pin. They are opposite ends of the same wire, but that wire carries a lot of current.
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