You left out wattage and voltage on the LED string, knowing these 2 could help,
as its a "LED bulb" I am going to guess it will be in the ballpark of 7-15W electrical power used,
Are you sure you cannot get by with a mains buck converter, rather than a 40V limited one?
In the past i may have just gated a thyristor on the tail end of the waveform to keep the rectified voltage low, but this looks a bit ugly on emissions tests.