Stated sensitivity figures are usually pretty misleading. If you calculate sensitivity for the 12LFC it's 94.6 dB/W/m vs 96.6 dB/W/m for the 12CL76. Regardless, HR will probably tell you that the 12LFC has higher sensitivity from 75 Hz and up, plus a smoother response. Yet, I would rate the 12CL76 as the better driver here and if the extra money isn't an issue, I would choose it over the 12LFC.
As for simulating in HR: The internal width of Cubo 12 is 42 cm, however at the beginning of the horn there is also a bracing of 1.5 cm, so effective internal width is 40.5 cm at that point, times roughly 4.2 cm at the beginning of the horn, 4.2 x 40.5 = 170 cm^2 (= S1 = S2). See also the explanation in this post: forum.speakerplans.com/guide-to-winisd-p...post21610.html#21610