You can't have it on by default. What I gather is that since most files are not "executable" the default setting is 0666. What option you do have is to set the umask value in your /etc/bash.bashrc file. The umask value then can be subtracted from this increasing security. You could set to 000. So at best your files by default will have read/write permissions for User , Groups and Others.
Another option might be to write a script to run as a cron job continuously ( or maybe every 15 minutes or so,you may not want to chew up resources)
chmod 0750 /path/to/target/*
Then set up as a root cron job:
m h dom mon dow command
* * * * * /path/to_your_script/Script
Hope this helps.