You can't do it with CM. You have to edit the data.acd file. Here, i've replaced the tyres on McLaren and Benetton for you to try. Also reduced the weight as others said. Both cars are now aproximatly 3-5 seconds a lap faster depending on the track. In 1991 qualy pace was 5 seconds faster than race pace, so this will put cars closer to qualy pace (depending on track, somwhere even exactly at that pace), but the cars are now too fast for race pace. As you wish. If you don't like it, I included original data.acd file in seperate folder so you can go back. If this is something that you like, I can make all the other cars for you. Remove .txt behind the file and extract to correct folder.
This a test qualy at Spa 1993 (Bennetton and Mclaren on RSS tyres, Senna and Piquet had some traffic I guess, but you can still see the difference):