1   On the intake manifold, how many sensors I need space for, and size of
the threads? I'll use the EM2.

I have 3 1/8 npt threads in the middle of the intake plenum. 2 for Manifold
pressure and one for the EC2 intake temp sensor.

2   The standard Mazda oil cooler, does it need the thermostat removed and
the bypass plugged? My WP has no thermostat.

I left the thermostats in the oil coolers

3 For the water radiator I'm planning of using a core 3 times the size of
the oil cooler. Does anybody think that it will be too small?

You've seen mine. Its big at 23 * 16, but it cools without a lot of scoops.

I just hope the engine can spin it?

It'll spin. Question is - how fast :)

John