Pops,
To flesh out using the gland nut and flywheel prop flange some more. The 4130 larger gland nut options seem like a no brainer.
Will either a 42 or 44mm one still work when using a GP prop hub extension? Without having one in hand it is impossible to figure out the height of the nut, or if a larger socket will fit in the counterbore of the flywheel/ dowel pin area, or not protrude and interfere with the prop extension.
Also, with an 8 pin crank it would seem like a bigger nut is better. It also looks like GP has a regular bolt of some kind for theirs. Do you know offhand the thread size for the gland nut? It measures out around 29mm or so. I have attached a picture.
Hub ID is 2.293 with a shoulder depth of .146 Flywheel center counterbore is 2.570 and .648 deep.
Thanks for helping us understand this.