Generally speaking, the primaries go to the front, secondaries to the rear, though this could vary depending on what carb you use as some can be mounted with the primary/secondary positioned at 90 degrees from the norm.
On the spacers, what you're mainly looking for is keeping the carb cool. You might gain some torque increase, but unless the spacer is rather tall, the effect won't be much. The idea behind it is that the spacer gives a "ram" effect to the incoming air/feul charge, think of the old Sono-Ramic and Max Wedge Chryslers from the '60s. It's the same concept. the trick is tuning the spacer length to the engine dispacement and the manifold type to get the effect and power gains. Basicaly put, the longer the runner length, the more torque is made.
I hope this information helps.
p.s. The floats bowl does usually go to the front, exept on some Rochesters and drag race type Holleys.
