I think this is the right way to go about a conversion -- leave the adapter plate larger than needed, and go with a rectangular shape.
After everything has been fitted to perfection (your shop's, that is) the plate may be scribed to the transmission (there are several variations, so no one adaptation is likely to be identical to the next) and is cut using a bandsaw just outside of the line, and is thereafter ground to the line, using, say, a combo 12" disk and 6 x 48" belt finishing machine.
As most know, I am partial to preservation of the dc drive, when feasible, but with the advent of sensorless vector drives, and vector drives which have a provision for tachometer feedback, in an affordable 5 HP size, possibly also single-phase without derating, conversions are becoming more and more the path of least resistance.
I am still not convinced that threading of the shaft extension is a requirement. Rather, a light press fit, made permanent with Loc-Tite and a tapered pin, should be more than sufficient.
After all, the extension is not supporting much of the input gear, that gear being supported mostly by the actual shaft, not the shaft extension.
However, for more than practical reasons the shaft extension has to be machined for the key, or the input gear's internal key has to be truncated so it won't engage the extension.
I was scanning some "ePrey" auctions for suitable 5 HP motors, and one in particular featured a 1-3/8" shaft.
Usually, 1-3/8" shafts are long enough without having to deal with an extension, but motors with those shafts are rare.