Can I use a transformer for a 240v stove/oven on a 120v outlet?
6000W at 110VAC would equate to ~54.5A. It would be extremely unusual to see an 110VAC outlet supporting that level of current. You really need to show a picture of the outlet to allow members here to verify that this is a compatible outlet. 

A 55A circuit would require feed wires of at least 6 awg copper wire. You need to show evidence that the "really beefy wiring" is actually connected with 6 awg copper or larger diameter. In addition we need evidence that this circuit is being fed from a circuit breaker not exceeding 55A.

If your power feed is indeed rated for 6000W at 110VAC then a transformer (assuming 100% efficiency which they are not really 100% efficient) would not be able to convert to more than 6000W at 220VAC. This would result in a maximum converted current not exceeding ~27A. Clearly this is not enough to supply current on a circuit to a stove that requires a 220VAC feed at 40A.