If your cows are in very good condition, I would suggest you give the shots in the lower quarter if you cannot give it in the neck, just to insure that you are injecting into muscle and not into fat. This is usually not a concern, but in the event your cows are in high rig, make sure you are injecting into the muscle. And yes, I would agree with using a 1.5 inch needle. It doesn't hurt the cow any more than a 1 inch needle.
Many donor cows that are fat, have poor responses because the injections are put into fat. The absorbsion from the fatty tissue is much lower than it is from muscle.