Yes, I also agree with ZooCVT, but again, I wanted to answer the question that was posed about the Benadryl, as that is what you had asked specifically about.
Surely, you would want to ask your own vet about what would be the best medicine to use in the case of your dog, and with the long ride, and the dog being very young, it is best to follow the advice of your vet.
Depending upon how long you have till you are taking this long trip, you might also want to look up "Pet Sedate" by Francodex, that helps with calming down dogs and cats. I have used that with success for my animals. It can be found for sale on the internet, and is not very expensive. It is herbal, and is easy to use.
God bless and good luck with your journey!
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Elena Marie 
"Open your heart to learn all you can in this life, and to give to others, the joy and satisfaction that they may remember you as a true friend."
