I think Reagan still would have challenged him in 1980, and maybe Bush also.
For the Democrats, I'm guessing the field would have been similar: Kennedy, Brown, and perhaps Carter and/or Udall would have given it another try. Senators Glenn and Hollings might have made a move 4 years earlier if a Republican were in office instead of Carter.
The results would depend on things such as how well Ford could work with Congress (O'Neill evidently hated Carter, whose lack of contacts in DC in general didn't do him any favors), whether or not the Iranian hostage crisis happened, in addition to what you suggested about the economy.