Maybe a winter project, but not something to rush towards. If you like your current SL frame, you might get the swap done and find the SLX has been ridden harder or assembled on a Friday, or the phase of the moon was so and so, and it may not ride like your current bike. Then again, it may be your dream ride. Only way to know is try it out but it's not that much of an upgrade, and since it's probably at least 35 years old, age and wear may make much more of a difference.
Phil