Hmm.. that's a setup I wouldn't be able to answer about with confidence.
1) I can't be sure I understand your setup from text correctly
and
2) Alternating blocks isn't something I've tested anything about
For a setup like (looking from above):
Stone Wall - empty ----
Stone Wall - stone - farm - stone - farm
Stone Wall - stone - farm - stone - farm
Stone Wall - stone - farm - stone - farm
Stone Wall - empty ----
Looking at it without a wall at the end of the stone lines (ie., no support from the ends), the first (row of) farm plot(s) will collapse first, and it would collapse the moment you try to attach the second farm plot. In other words, there's no benefit from that direction.
If the stone block lines are attached to a supported stone wall at the ends (as in, properly supported from that end), you should be fine with a length of about 12 - one stone row carrying one plot row, while being carried from both ends. If your vertical space is limited, that should work;
Stone Wall - SW - SW - SW - SW -
Stone Wall - stone - farm - stone - farm
Stone Wall - stone - farm - stone - farm
(repeat for 8 more rows)
Stone Wall - stone - farm - stone - farm
Stone Wall - stone - farm - stone - farm
Stone Wall - SW - SW - SW - SW -
I wouldn't build a massive thing without trying in a test world first, the SI calculation may well do a hiccup at some point. Having a solid floor would make it more stable, but if it stays, it stays ...