First time owner setting up their tank..

In my experience it's definitely possible, even for a beginner but maybe that's just my experience (I only have one snake and therefore am not the most experienced person).

I think that it's important to take a few things into consideration. In my opinion 40 gallons is not that big and you don't need to move your snake immediately into the big enclosure.
If I was you, I'd do the following steps:

1. Buy a smaller plastic box and make sure it has holes for the air flow. Cover the bottom with paper towels. Choose a spot for a heat mat that you put under the box with a thermostat attached to it. Put a hide over the heat mat and one on the other side. At last put a water bowel in there (and maybe something to climb on).
--> This would be a setup for quarantine. I know many people don't do it with their first snake but I personally think it's important. You want to look for snake mites and weird behaviours. Personally I did two fecals with my snake once I got him and I decided to do one every year.
Depending on if you have other animals I would quarantine the snake for at least 1,5 months or 3 months.
--> (In the beginning) it would also solve your problem with the big enclosure. Your snake will be bigger when he'll be moved into the bigger enclosure.

2. The bigger enclosure must be crammed. Use a lot of plants and branches to make him feel safe. You shouldn't be able to see some parts of the enclosure. Be sure to cover the sides of the enclosure.
If you do that, he should be fine with the enclosure.

Personally I had a quarantine box in which my snake stayed for 1,5 months (please forgive the weird numbers but we don't use gallons as measurement but I converted the measurements we use to gallons) that was 36,9 gallons (he was 4 months and 100g then), his second enclosure was 46 gallons (he was 5,5 months old and weighed 212g when he moved) and the one he lives in now is 210 gallons (he is 1 year and 4 months old and weighs 820g).
I never had issues with even one of these enclosures. He went on a 2 months fast in his newest enclosure but in the last month he has started eating again (four consecutive meals at the moment).
So I had a great experience with big enclosure even with little snakes and I am also a beginner.