Alright, this, finally, was my original post. You can find more info here:
You can change any car to work in any slot.
1. Change the name of the vehicle files:
police.yft ----> police3.yft
police.ytd ----> police3.ytd
police_hi.yft ----> police3_hi.yft
2. In OpenIV, export your Vehicles.meta file from (mods folder) Update -> Update.rpf -> Common -> Data -> Levels -> gta5
-To edit it, Notepad++ is a great free program.
-Find the vehicles.meta entry for the original model the car was named after (in the example above, this would be police).
-Copy the entire entry (from <Item> to </Item>) and paste it above the top entry
-Change the model and texture names of the vehicle to police3:
<modelName>police</modelName>
<txdName>police</txdName>
<modelName>police3</modelName>
<txdName>police3</txdName>
-Now find the old vehicles.meta entry for police3 and delete the entire thing.
-Save and replace your Vehicles.meta in OpenIV.
3. Do the same thing for the Carvariations.meta.
-There are many different carvariations files, I use the one in (mods folder) Update -> x64 -> dlcpacks -> mpvalentines2 -> dlc.rpf ->common -> data
-Export with OpenIV and edit with Notepad++
-Copy the carvariations entry (from <Item> to </Item>) for any vehicle. They're all formatted the same, it just decides what colors the car spawns with.
-Change the <modelName>police</modelName> to <modelName>police3</modelName>
-Near the end, change the <sirensettings value ="1" />. The vast majority of custom cars use the value 1.
-If they use a special sirensettings value, it's usually in the description, or a custom carvariations entry is provided.
Make sure you've replaced your edited vehicles.meta and carvariations.meta files in OpenIV.
The car model files can go in one of the DLC patches:
(Mods folder) Update -> x64 -> dlcpacks -> patchday11ng -> dlc.rpf -> x64 -> levels -> gta5 -> vehicles.rpf
*as new dlc patches come out, cars can be placed in the latest dlc pack; i.e. patchday12ng or anything lower on the alphabetical list. Putting your cars in the latest dlc pack just ensures that they will overwrite any previous files, such as those in x64e.rpf or any other dlc packs.
Even though there are no police3 model files in patchday11ng's vehicles.rpf file, your models will overwrite police3 because the game loads this dlc pack after any other files. Hence it's a good idea to just load up your cars into the lowest dlcpack in the list found in (Update -> x64 -> dlcpacks).
As long as you don't mess up the meta files when you edit them, you can change any car to any slot using this process. The key point to remember is to copy the vehicles.meta entry of the car slot it was originally built for (police), and change the model/texture names to the slot you want to put it in (police3), and overwrite the intended slot's entry (police3). Same for carvariations, except you can use any entry and edit it as described above.
This method works for adding new vehicle slots as well. Simply follow the steps above, except anywhere you would put police3, put a custom name like dodgecharger instead. As long as you have 1. a vehicles.meta entry 2. a carvariations.meta entry and 3. vehicle model files (yft and ytd's), all with the same matching names, you can add cars to your game without overwriting slots.
This process seems a little long written out but once you get the hang of it it's not so bad.