You have two legs of power, X and Y, and neutral N. Power all of your 120V components off of leg X. Feed your SSRs with leg X (I assume you have one SSR per element and are only switch one leg, correct?). The output of each SSR will feed one terminal of each element. The other leg of the element will go to the common pole of an appropriately sized DPDT switch (or relay controlled by your BCS...). One switched pole of said DPDT switch will go to N for 120V operation. The other pole of said DPDT switch will go to leg Y for 240V operation.