The first consideration to make is will the working volume of the furnace allow you to physically load an additional 350 pounds? You will need to be careful not to load the furnace with so much work that you reduce the air circulation through the workload.
The formula for calculating the additional electrical energy is as follows:
KWh = [Gross load (including trays, fixtures, etc.) X specific heat X temperature rise from ambient temperature to maximum operating temperature] ÷ 3412 (Btu in 1 kWh) (constant)
This will give you the total energy required to heat up the gross load only in one hour (it does not account for any heat losses whatsoever). Heat losses will occur through openings, feedthroughs, thermocouple holes, air-circulating fans, door seals and of course refractory insulation. With approximated losses at 35%, the calculated value can be divided by 0.65 to better approximate the necessary energy.
Once this value has been calculated, it will be necessary to calculate the element size and rating. Can this new element size with the necessary kilowatt rating fit onto the furnace wall? If the new elements cannot be fitted onto the furnace wall, it will be necessary to determine if the workload can be raised to the process temperature in a longer time. Then you would need to incorporate the time to arrive at the final value.