the pillar housings may not be perfect either (unless you've seen them, to know), in terms of fit and colour.
the A-pillar comes off easily - it is clipped into place. if you have curtain airbags, there is also a retaining strap near the top, secured with a 5mm Allen key bolt which you simply undo.
with gauges mounted here, also be mindful of how you run the wires. for neatness, it may require drilling holes in the trim (also to secure the pods). that is fine if they will be mounted permanently, but if you wish to sell the car without them, you'll need a new trim in the future (maybe buy a spare now, say from a wrecker or car that is being 'broken' for parts).
GND wires i would connect directly to the chassis below. there should be a factory ground bolt in the kickpanel area. lift edge of carpet and remove door sill plastic trim panel.
POS wires depends how much current the meters will drawer. presumably not much. i'd use the CIG as the power source.
not sure about the 147, but the ILLUM powerwire to the CIG in my GT is only +3V, and not +12V. may or may not be strong enough to trigger the lighting in the meters. whilst the GT has another plug with an ILLUM wire, it 'floats' at +3V then goes to +12V when the lights are on; 3V is used instead of 0V. it may suit your gauges, but it does not suit my alpine headunit, which is quite sensitive, and dims the display with 3V (and may trigger the lighting in your gauges).