No reason why you cant mount it above the exhaust manifold, although some heat sheilding (ceramic coat exhaust?) would benefit cold air induction.
No you cant simply stick another pulley on the water pump, the vee-belt driving the water pump from the crank will slip like all buggery when the supercharger is running. There is a reason why Toyota use a 5 rib serpentine belt to drive the SC14.
I recall reading about the use of a 2Y bottom pulley (crostek posted) somewhere in this forum.
Have a play on Desktop Dyno etc and see how achieveable 300HP is.