It's fun to debate how great athletes from a prior era would perform today if they were still in their prime. The problem is that with all the changes in rules, sports medicine, and strategy over time, it's hard to determine what kind of stats they'd put up. But I figured I'd try anyways and in this case, I picked the great Joe Montana as my guinea pig.
In 1989, Montana led IMHO the greatest Super Bowl team in NFL history and posted some impressive stats along the way leading the NFL in passer rating and yards per game. Considering that the average passer in 1989 threw for 21 touchdowns, 20 interceptions, and 3374 yards, Montana's stat line of 26 TDs, 8 INTs, and 3521 yards looks more appealing.
So how would 1989's Joe Montana fare today? I used factors and league averages to estimate. In 1989, Montana threw 25% more TDs than the league average, 60% fewer INTs than the league average, and 4% more passing yards than the league average. He did all that while playing in just 13 regular season games.
I applied those factors to last year's league averages which were 24 TDs, 15 INTs and 3700 passing yards. So using 1989 metrics, a 2012 Joe Montana would post 30 TDs, 6 INTs, and throw for 3861 yards. And if you pro-rate those figures over the course of a 16-game schedule, he throws for 37 TDs, 7 INTs, and 4752 yards.