Toroidal transformer
This is a tricky one. The max rating of the BLW77 is realistically 35 volts but lowering the voltage reasonably under 35V to (say) 32 volts by a resistor is going to dissipate a lot of power in the resistor at 8 amps current.

Power in resistor will be V.I = (35.7 - 32).8 = 29.6 watts. 

Dropping it to 28 V is going to dissipate about 62 watts so I believe your best bet is to construct a buck regulator to drop anything between 30V to 40V down to a regulated 28 volts. Maybe one of these: -



You can set the output voltage to be 28V by adjusting the potential divider on the Vfb pin. For alternative try Linear technology and use their technical search engine for narrowing down results. TI also have a great range of buck regulators too.