I would suggest the following;
1. The minimum preheat would be 500 deg F.
2. Because of the hardenability of 410 stainless, and lack of ductility associated with Stellite 6 hardfacing, you will need to deposit a butter layer of Ni-base (Inconel 82) prior to depositing the Stellite 6.
3. I would suggest two layers for buttering followed by PWHT. The PWHT temperature should be either 1400-1450 deg F or lower, if the 410 stainless steel was heat treated (quenched and tempered). The PWHT temperature should not exceed the original tempering temperature, if applicable.
4. Perform a Liquid PT of the butter and surrounding base material to assure no defects.
5. After PWHT, preheat to 500 deg F and deposit the Stellite 6 on top of the butter layer using two layers to ensure proper hardness (no dilution with the butter layer).
6. Perform a liquid PT after machining.