You do want to stick with the 1 1/2" needle as the further into the muscle you are, the less painful it is.
You can use a 22g needle as well. I use either 22g or 20g, whichever I have on hand, but the 22ga is noticeably smaller.
It's useful to have cross ties and a person holding the horse to do injections. I simply use cross ties with my horses and they stand well for injections. Make sure you also know proper technique as that will decrease the amount of pain the horses feel
They say money doesn't buy happiness -- well happiness doesn't buy horses!