if anything the democrats should have gained MORE seats in 1982 to get to the 290 they had in 75-78. You have to remember that unemployment was eleven percent by election time. That, plus, democrats controlled redistricting in most states.
The only two significant states Democrats controlled redistricting in were New Jersey and California. They controlled most of the south except Virginia, but they were already so dominant there that it didnt matter. Everything else was pretty much split, while Republicans controlled Pennsylvania and Indiana.
There was no way Democrats were getting to 290 seats again. Redistricting had actually helped some Republican incumbents who could have lost and to get to 290, Democrats needed to win some what were then overwhelmingly Republican suburban districts that they were only competitive in in 1974 because of Watergate.
I could have seen them getting possibly to the mid to high 270's as there were seats like AL-02, IN-03, MO-07, PA-21, OR-05, OH-12, CT-05, ME-01, and VA-08 that Republicans barely took over or held onto.