Won't work. The Signet's lens has to cover 24 x 36 mm, i.e., a 43 mm circle. It probably just covers that. Lenses for 5x7 have to cover 210 mm, for 8x10 have to cover 300 mm.

There's a reason why LF shooters don't use lenses for small formats, and it isn't love of spending more than necessary.