While modifying the config.php to ignore permission checks is not recommended, there might be a way around it if you understand the risks.
However, editing the config.php to skip permission checks is generally not recommended because it can lead to security vulnerabilities. NTFS file systems don’t support Linux file permissions like 0770, so ownCloud may not function as expected.
I would advise against setting permissions to 777, even on a closed network. There could be potential risks you’re not considering. A better approach would be to set the folder permissions to 0755 and chown the directory to the user running the webserver process (e.g. www-data). This would give the webserver user ownership and permissions to read, write, and execute the files while all other users would be restricted from write access.