Hi Steve,
I apologize for the delay in response and appreciate your patience.
1.) Have you tried increasing the DTC resistor value in your design? I saw that you said you've tried increasing deadtime, is this resistor value what you've adjusted?
2.) Have you tried adding 10k Ohm Pull down resistors on the low-side Gates?
3.) Does increasing the high-side gate resistors to a value around 40 - 50 Ohms decrease the voltage spike measured?