How many watts max at my 220v L14 30R outlet?
0 watts.  A 30A receptacle on a 40A breaker is a code violation. 

5760 watts continuous, 7200W intermittent, if you change the breaker to 30A.  

7680 watts continuous, 9600W intermittent, if you change the receptacle to NEMA 14-50R. 

Why are we using a NEMA 14-50 on a 40A circuit? They don't make 40A receptacles/plugs.  You are expected to use 50A sockets on 40A circuits. This exception is coded into NEC table 210.21(b)(3).