Some things I incorporated for classic teams...
For the QBs, you really have to raise all the Passing attributes into the 90's. The Pass Read Coverage one is key and should be between 95-99. In addition, Composure should be raised into the 90's as well. The superstar QBs should be between 95-99, but even the lesser QBs should be above 90 for all the passing attributes. Remember, these are classic teams and most of them have been in or won Superbowls/Championships. So you are dealing with the cream of the crop here.
WRs need their catch ratings raised too. NFL players should all have catch ratings from 85 - 99 for WRs and RBs. Thier Run Route ratings should be upped about 10-20 points as well.
Adjusting the DBs means lowering Agility (how fast they close in on the receiver) about 20 points, make their Catch rating 0, make their Aggressiveness rating 0 (10 for Safeties), and lower their Coverage rating 10-15 points. This helps open up some space between receivers and DBs.
Linebackers should have their Catch rating 0 and Jump rating 40. Their Coverage rating should be lowered 10-20 points too. This also helps open up some passing lanes between the receiver and defense.
All of these changes go hand in hand with whatever sliders you are using as well. I use a set of sliders that have Passing at about 32 and Catching maxed out at 40. The defensive sliders are basically all 0, except Pursuit is at about 6. Sliders are a personal thing and all based on your skill level of playing the game. I tend to play Coach mode only so these kinds of settings work for me. But they may not be right if you control the players, so you have to experiment a bit.
C