hmm, Franconian as a C20/21 dialect area is not west of Corvey in Nordrhein-Westfalen. It's slightly east of Corvey, really, because it's northernmost Bayern and Baden-Württemberg as political entities go (Würzburg, Heilbronn). Sankt Gallen is across Bodensee from southernmost B-W; Reichenau (where things were also written down earlyish, though mostly just short glosses on Latin) is the near-to-modern-Germany side of Bodensee, also B-W. Middling B-W is my heritage dialect, which is why I have opinions. (Learning alt- and mittelhochdeutsch was far easier than it should've been, given my modern German's imperfections even then, because the vowels were familiar right up till we got to late mhd and moved northward.)

Anyway, if you drew a couple of horizontal lines across a map of Germany to include upper Bayern and B-W, then extended them westward, if anything you'd be slightly north of Strasbourg. We don't have proto-French attested that early outside the Oaths (most stuff was written down in Latin until insular French becomes a thing tempore Chanson de Roland and then bleeds back onto the Continent), so I'm a little confused by the "more western forms" part of the question.