.290 duration sounds fairly large, but that depends on the LSA. I'd like to know that before I could really comment. A wider 110-112 LSA wont be bad, but if it's less, that .290 duration will really be a battle with the TH350, even with 3.73 gears. Some 186 heads have 1.92 intake valves, while others are 2.02" valves. The 2.02" came unshrouded, and if yours are unshrouded they flow decent. Some with 1.94" valves were converted to 2.02", but not unshrouded like the factory, and they don't flow as well.
That 750 is a huge carb, and I'd be inclined to hope the Demon is a 600 or 650 cfm, as it would work much better with an automatic. Hopefully your tires wont be too tall, and cancel out the 3.73 ratio. A 26" would be good, and a 28" tire would be pushing it with the 3.73 gears and a TH350 first gear.