where to update nginx config?

I have a Laravel 7 site running on Forge. ITs been doing fine but in the last couple days had some 502 errors. While reviewing my nginx logs I saw a bunch of:

2022/06/06 22:39:04 [error] 9077#9077: *1790120 upstream sent too big header while reading response header from upstream, client: 74.71.32.109, server: [URL], request: "POST /login HTTP/2.0", upstream: "fastcgi://unix:/var/run/php/php7.3-fpm.sock:", host: "[URL]", referrer: "[URL]"

I reached out to Forge support about this and they routed me to: https://www.digitalocean.com/community/questions/nginx-returns-upstream-sent-too-big-header-while-reading-response-header-from-upstream. I had seen much the same notes in my own googling.

My current nginx config is:

# FORGE CONFIG (DO NOT REMOVE!)
include forge-conf/{{ SITE }}/before/*;

server {
    listen {{ PORT }};
    listen {{ PORT_V6 }};
    server_name {{ DOMAINS }};
    server_tokens off;
    root {{ PATH }};

    # FORGE SSL (DO NOT REMOVE!)
    # ssl_certificate;
    # ssl_certificate_key;

    ssl_protocols TLSv1.2;
    ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384;
    ssl_prefer_server_ciphers off;
    ssl_dhparam /etc/nginx/dhparams.pem;

    add_header X-Frame-Options "SAMEORIGIN";
    add_header X-XSS-Protection "1; mode=block";
    add_header X-Content-Type-Options "nosniff";

    index index.html index.htm index.php;

    charset utf-8;

    # FORGE CONFIG (DO NOT REMOVE!)
    include forge-conf/{{ SITE }}/server/*;

    location / {
        try_files $uri $uri/ /index.php?$query_string;
    }

    location = /favicon.ico { access_log off; log_not_found off; }
    location = /robots.txt  { access_log off; log_not_found off; }

    access_log off;
    error_log  /var/log/nginx/{{ SITE }}-error.log error;

    error_page 404 /index.php;

    location ~ \.php$ {
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass {{ PROXY_PASS }};
        fastcgi_index index.php;
        include fastcgi_params;
    }

    location ~ /\.(?!well-known).* {
        deny all;
    }
}

# FORGE CONFIG (DO NOT REMOVE!)
include forge-conf/{{ SITE }}/after/*;

Where would I add the proxy_buffer_size configuration?

    proxy_buffer_size          128k;
    proxy_buffers              4 256k;
    proxy_busy_buffers_size    256k;