if you measure outside of bead lip to outside of bead lip, they'll be about 6" wide. If you measure the inside bead that actually supports the tire, they'll be 5" wide. KH disc brake wheels for the same car will be 5.5" wide with a 15 X 5.5 stamped in the wheel band and maybe even the original paper label saying the same thing. If you're lucky to find one in good shape... The disc brake wheels are a half inch wider than regular fullsize wheels.
As far as police cars that might have wider rims, The 3-4 67 police cars that I have parted out were all disc brake cars so had proper disc brake 5.5" wide wheels. None of them had 6" wide wheels. I'm pretty sure if you measure disc brake wheels outside to outside, they are about 6 1/2" but they're still skinny wheels- not like a true 6" or 7" wide rim.
I cannot say there were no actual 15x6 (measured bead-to-bead) wheels supplied as oem on a 65 Galaxie in some configuration but that chart is just Internet fodder for the sheeple....
1967 Galaxie 500 Red original Q code 428
1967 Galaxie 500 White with Black stripes 428PI clone- car is dead & long gone, sold near Oak Ridge TN 2000, would love to find it.....
Please visit http://carolinasculpturestudio.com/
for pictures, information and video links to my work.