Well, this is somewhat interesting and different from all the mods ive ever done (like the transformers)
its something i actually have wanted to do in the past but lacked the skills on 3ds max to do so.
for now, ive replaced every police vehicle i could remember with other default ones
Sentinel for LSPD
Stratum for LVPD
Sultan for SFPD - slightly modified.
Wayfarer for HPV1000
Yosemite for Rancher
Monster Truck for FBI Rancher - heavily modified version of it, made it 4 door with back closed, LSPD markings cause i found it interesting to have one more vehicle in their fleet.
all of them have GTA V police cars interiors (all but the suvs are the same, i'd like to work on different for each but that would be time consuming) and also feature an old 870 with wooden parts converted by me (low quality version of it)
For now im thinking on also converting the patriot into a police version and giving it LVPD markings, and not sure about a SUV for SFPD (probably not, cause its mostly city'ish, not like LS with woodlands around or LV with the desert)