The NA6 sending unit is HUGE. It sends out varying resistance throughout the pressure range.
The NA8 is just a switch that's set at 8 psi (I think) and is comparatively tiny. When it reads below the set pressure, the switch closes and grounds the gauge, making it drop to zero.
As far as I know, the windings are backwards. Watch the video and see the behavior of the gauges when power is applied. They seem to be backwards from each other.
The NA8 also has damping grease.
So... in theory, you could convert one gauge to the other by rewinding it, removing the grease, and changing resistors. OR you could take advantage of the cheap and plentiful NA6 gauges (they're usually around $20 each) and save yourself a ton of time.
The expensive item is the NA6 sensor. You can still buy them new, but they're rather not-cheap and I'd personally not trust a used one.