Don't want any fires, and I have a 430w HPS ballast and a 400w metal halide conversion bulb.
Is this unsafe? If so I will just use the 430w sodium bulb for now.
Thanks

Probably not a good idea. It will shorten its life and probably get too hot.

I have a xtrasun 430 Watt HPS ballast. The grow store included a Solar NRG. (400 Watt HPS to MH conversion) After 3 months of using the bulb we moved. The bulb worked for 2 days at the new house, and then went to just a blue arc. The move may have negatively affected it, but it could have been the ballast. I've heard they are nearly identical components, but those slight differences affect it.