This Gaping Maw

Working on this 1934/35 Regal built Dobro. It's a Model 37 roundneck in with a few issues-main ones being a bodgy slotted headstock repair, some missing binding, misaligned tailpiece, modified rim on the soundwell etc. It's been a gig pig on and off for the 30 years the current owner has had it. He want's to ditch the Barcus Berry PU, which was on it when he bought the guitar, and patch the hatchet work. I'm guessing the top was originally cut to accomodate a larger PU?!

Currently thinking that to reinstate some stability I will square up the cut a smidge and glue a 1.5mm-ish maple backing plate that extends maybe 15mm north and south under the top. Then glue in top patch of seasoned birch ply or solid maple. Would it also be worth shimming up the neck stick minimally to widen the gap a tiny bit to build some pressure into the glue ups when shim is removed later? Any input or suggestions most welcome.