The only thing that should force a top or bottom mount are clearances to the firewall, engine mount and steering rack. Not a big fan of spaghetti tubular headers myself... questionable performance benefits over an OE log-type manifold, and you should be able to adapt one for the flange of whichever turbo you choose. Different OE manifolds have different outlet flange positions - some hug closer to the engine block than others, which is what you want if maintaining a bottom mount is desirable. For accessibility (not to mention appearance), most people opt for the top mount, which also places the oil return drain at a more favourable and necessary position above the corresponding hole above the oil pan. I've even seen some builds with these big turbos straddling over the gearbox for space.
I prefer the GTB3576KLNRV myself, but they are harder to come by than the BW S356V, and the latter has many fans and users here. A single turbo setup with one of these big boys will benefit from the quicker spool thanks to the ball bearings in the former, and even better if you can adapt to directly use an electronic actuator off the N75 PWM signals without converting to a vacuum actuator.
