Last of the Shovel Heads, First of the new style 5 speed rubber mount frames. If you don't mind a little Fab work, Lepera is a damn good bet.
Just made a Lepera Monterey w/ Pillion Pad work on and 89 FLHT. The Seat was for 91 - 96 FLHT's, but a little tweaking made it right.
It's worth giving them a call. They will send you a base plate with foam and no cover. You can then tweak the base plate and foam to your specs and then they will cover for you.
83 to 89 frames are damn similar to 91 - 96 FLHT frames. It's the 97 to Present frames that differ greatly from the previous series frames in the seat area.