What opening should I play against an 1800?

(First time posting, please tell me if in the wrong section).

I'm playing in a scholastic tournament tomorrow (3 rounds, g45 d5) and it's likely that if I do well I'll have to face him. As white, I like to play the King's Gambit, and as black I like to play the Caro Kann, but considering the gap between our ratings, his being 1800 and mine being 1350, I'm not sure if I should switch it up and play something different, to give myself a better chance against him.

Play the King’s Gambit, favorite opening is best.

I agree, play the opening you know best. He will be nervous playing a lower-rated player, so won't like seeing the King's Gambit on the board. For future games, though, you might like to think about the balance of your repertoire. KG and C-K seem a funny combination. one very tactical, the other more positional - maybe learn a Sicilian?

The main reason that I decided to go with the CK and KG was to have an advantage in knowledge of opening theory, so I'm thinking of changing my opening as black to something else, but I don't know any tactical openings for black that a strong scholastic player wouldn't have prepared against.

Play what you know. Likely your opponent will try to pull some cheap tactics and not take you seriously. Look for a chance to punish their arrogance.

Also there is no way an 1800 can prepare for everything. Sure they probably have some idea of what to play against most things but their knowledge of theory is certainly not THAT profound so try not to worry about it.

If you know the openings this particular person likes to play you could prepare something specific against him. Otherwise just stick with what you know and are comfortable with, and be sure to focus on the game and not who you're playing. Be cold and objective

You should play what you know how to play best. It's your best chance.

Openings dont decide games at that level.

King gambit is nearly a suicide against strong player. 

Caro Kann is a solid defense against any level. But how deep you learned about that opening?There are a lot of struggles black has to sort out in opening.  People who know opening 10 moves deep will outplay against people who knows 5 moves deep. And 20 moves deep will outplay 10 moves deep.

Just example of Caro Kan two knights where I did not prepared well. I played a strategic mistake at move 5 .    d5??  ( horrible). I thought d5 would not be a big problem after queen exchange ( I underestimate the power of his light square bishop and potential open "f " file.)

At move 16.f4. I am totally dead positionally, the strategic mistake I did in move 5 proved that I played horrible.

If you enjoy attacking chess, Smerdon's Scandinavian is a great choice, and will catch a lot of players out.

I used to play that exact same combination! King's Gambit as white, Caro-Kann as black.

About 6 months ago though, I switched to playing the Sicilian Najdorf as black instead, and I'm enjoying that a lot more. I'd recommend you try it out. Having said that though, when you first start learning the sicilian, you'll probably lose a lot of games very quickly. It's one of those openings where you can get crushed in the first 15 moves if you don't know your theory. So in the meantime, I'd definitely recommend playing the opening you're most comfortable with.

Regarding the big 400+ point rating gap: sometimes good players screw up, since there's a lot of pressure on them to pull a win, whereas there's less pressure on you because you're expected to lose the game, so use that to your advantage.

Play what you know best. Trying to take a higher rated player by surprise by playing an opening you don't understand is a sure road to defeat. Specifically regarding the KG, no 1800 player is likely to be conversant with all the latest wrinkles. If he plays 1...e5, he has most likely studied the Spanish and Italian games for the most part.

Can the KG succeed against a strong player? Spassky defeated Bronstein, Fischer, Karpov, and Susan Polgar, among many others with it. Of course, few are as skilled as Spassky, but clearly even very strong players can have difficulty facing it.

Why do you play a risky tactical opening as white and a positional defence as black? Against a higher rated opponent, you should try to complicate the position. 

Nimzo-larsen attack. And from black, I suggest Nimzowitch defense.  You should play closed positional game. Players on 1700-1800/1900 are usually adept at tactics. To beat them, you must resort to positional and strategical approach.     

But people at his rating group are usually even worse at positional chess. The positional approach is what a higher-rated person should pull on a lower-rated. The mistakes glide right past you and by the time you figure out you did something wrong, you're dead.