0.3% carbon isn't too high, but preheat is a move in the right direction. The minimum preheat temperature is dependent on the thickness of the members to be joined.

You can't go wrong with using low hydrogen practices, i.e., a low hydrogen welding process, preheat, maintaining the interpass temperature, slow cooling. If you use the SMAW process, use E7018 that is removed directly from an unopened tin or has been stored in a heated electrode oven.

Use a filler metal with the lowest carbon content and the lowest tensile strength and lowest yield point to give you the highest ductility. In short, be selective. Review the manufacturer's product descriptions and choose the electrode with the best characteristics for the job.

The welded joint can be no better than the weakest link, i.e., most likely the ASTM A36.

Good luck - Al

Cast steel does have good weldability, but care must be taken in order to ensure that the welding process is successful and that the weld joint will meet all strength and safety requirements.
First identify type of Cast steel, like in your case, you have only provided carbon equivalent. Try to get full chemistry or grade. Accordingly you can choose a matching or suitable filler wire. Use this guide to find the right welding preheat https://www.materialwelding.com/what-is-preheat-in-welding/ temperature. Referring to the C%, you should not have any issue for the welding.