Done what you said and it fits perfectCheck the seat tube for ovality, especially by nudging the clamping area open a little. Also measure the post - it should indeed be 26.4. There can be some alarming discrepancies to what is nominal though, often 0.1mm.
Most years Lava Dome and Cinder Cone shared the same frame, so I would again start with 26.4.Done what you said and it fits perfect
Any idea what size a 91 Cindercones seatpost should be?