So as title says this is just a workaround just to let you play with controller and not a solution/fix to make controller be recognized by game (basically game will think you play with keyboard while you are actually sending keyboard and mouse inputs with controller)

1st of all choose your weapon

AntiMicro is FREE and very simple to use
https://sourceforge.net/projects/antimicro.mirror/
its easier to configure and easier to manage between profiles and different controllers

Xpadder is NOT FREE i think its around 10$ so either buy it or find it somewhere in the Southwest ((just dont go for version 5.3 or 5.7 which is free but i dont remember what was wrong with it but something was) im running version 2017.05.01 <- yeah literally it dont have version just date)
Its more complex in configuration NOT HARDER just more things to do
Main issue here is fact YOU need to manually create profile for your controller and arrange buttons (like physically place them somewhere on the screen AFTER u let xpadder recognize each one)
But after its done u wont need to do it again for same controller and doing it for next one will be piece of cake

here is tutorial for AntiMicro
https://www.youtube.com/watch?v=eCcu5eqdWds

and for Xpadder
https://www.youtube.com/watch?v=BL2sOwjgOrc

This is my button layout with explanation what individual button do but im very specific player so better change it to suit your needs

https://i.imgur.com/5t7JuN9.png

and here are original controls for keyboard and yes i got it only in that quality

https://i.imgur.com/6vGvH05.jpg

*** This is AntiMicro profile for Interstate '76 for ppl who did not went trough Controller Mapping in AntiMicro (pretty much your AntiMicro button layout needs to look like mine from screen above) (or in other words you skipped any configuration and just lanuched this profile)
http://www.mediafire.com/file/1fz10m815320261/Interstate_%252776.joystick.amgp/file

*** This AntiMicro profile for Interstate '76 is for ppl who did Controller Mapping and their AntiMicro button layout looks like the one in video tutorial above
http://www.mediafire.com/file/7c4c4zdls6gac81/Interstate_76_CM.joystick.amgp/file

This is Xpadder profile for Interstate '76 (some buttons most likely will be in wrong places)
http://www.mediafire.com/file/fa3naj04vgoyrgr/Interstate_%252776.xpadderprofile/file