Something in my OCD brain is screaming that this is Heath Robinson and fundamentally flawed, but I suspect that's a reflection on my mental impediment rather than your craftmanship. I overthink things, I balance tyres to 0grams, so despite my obsessive nature kicking in here, this will probably work. However, indulge me, how critical is the concentricity of that hole to the spigot shaft from the gearbox in the final conversion? Does the motor have a spigot that mates to that hole? Or is the hole deliberately oversized to allow you to centre the motor to the spigot shaft through the coupling, then just whack in some threaded holes in the right places on the adaptor plate? I'm not meaning to be an internet smartass I'm just worried you might tan the motor or gearbox if the alignment is out and you put some deflection on either or both of those components if this is a little bit out.
IRL, industrial machinery uses couplings between the motor and driven machinery that has a little bit of flex in it to allow for minor misalignment, but even if the coupling has a bit of give in it, the machine and or its motor have a shorter operating life if there is any misalignment present. For example, I once got tasked with working on a big centrifugal pump because it was rumbling, split the
Detroit Coupling* and checked the faces against the datasheet for the coupling, and it was well within official tolerances. So the decision was made to remove both the pump and its 88kw motor, on the bench both ran free with healthy bearings, so I put it back together again using its original shimming, and it rumbled... Long story short I reshimmed it to within a gnats genitals of perfection, reassembled its Detroit coupling, and it ran a dream. So even with a coupling with a bit of flex in it, the misalignment caused issues, if you're going to be running with a solid coupling Nodge makes for you by welding two female splined sections, turning them down in a lathe, and welding them together, there is potential for damage to motor and gearbox, so I'd urge you to think carefully about the alignment at this stage.
*Detroit coupling pictured below, prime mover gets one of the silver centre spigots with the splined faces on it, the driven machinery gets the other, they are aligned to almost touch but not quite, say a couple of mill out, but having a distinct airgap between them. The two halves are linked by the spring steel zigzag element, then the two outer shells are mated and pumped full of grease which is held in place by the black wiper seals.
