You can actually compare stats across eras...
Just normalize stats according to 2021 numbers and apply backwards. I did this years ago, expecting Marino or Manning to come up tops in stats. It actually wound up being Brady. I wanted to adjust for playing conditions as well, but that's a huge HUGE project and I don't have time for all that. An early look does not do Brees or Manning favors, but helps Brady, Rodgers, and even Jim Kelly a bit.
That said, Montana's 1989 season goes down as one of the best of all time. Brady's 2007 remains the best of all time. Manning's 2013 season doesn't crack the Top Ten.
Also, Sammy Baugh is criminally underrated. Completely dominant during his time. Staubach also doesn't get enough love.
In any event, Montana was great. Amazing. Brady was better. There are no objective measures to suggest otherwise.