My dog is 70 pounds and was given 300 mg of trazodone does this seem too much?

That seems to be excessive. Vets tend to over-prescribe dosage. I won't get into the reasoning, conclusions can be made. I'm thinking you are done with this med since post is 4 months. I hope he is doing well and no ill side effects.

300 mg for a 70 pound dog is a low end dose. The canine dosage is 1.5-15 mg/pound. That dosage is 4.28 mg/pound. That is a lower end dosage. NO. Your veterinarian didn’t overdose your dog. NEVER give your pet ANY human or OTC medications without being expressly told to do so by your pet’s doctor.

Trazodone is very commonly prescribed for our canine patients and often at doses that are much higher than the usual human dosage.
The dosage for dogs varies, but a general guideline is a starting daily dose of around 2.5 mg to 3.5 mg per pound. In some cases, depending on the injury and the dog's pain threshold, they may be given up to 15 mg per pound every 24 hours.
We use this medication because it has been shown to be safe and very effective for dogs (and cats).
WildcatVet