off-axis means you are closer to 90 degrees relative to the centerline of the woofer - i.e. it is aimed away from you.
on-axis means you are closer to centerline of the wooofer - i.e. it is aimed at you.
all speakers have substantial high frequency roll-off when off-axis.
you can put them in the dash (and lose visibility as well as cut the dash) or you can put them in the kicks. you have plenty of kick space once you relocate some of that crap down there (behind the kick panels). i vote kicks - plenty of room to make them work and give the woofers lots of airspace so the f3 point is lower. also, you improve ITD so you are less reliant on T/A