Question: Would it help on the real test if you train using a 35lb-40lb can?
Answer: It could help to do a can a little heavier than normal, perhaps 35 lbs, but training with the real thing will also be advantageous for you since the timing and rhythm will be exactly the same on the real test. A heavier can might be more appropriate for separate, general strength goals then test-specific training.
Answer: Correct–the size of the can is not going to be a significant factor. It will just be useful to get used to the awkward boxiness of the can. You can even just use a 30 or 35 lb dumbbell if that’s all you have.