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Keep in mind im building this for extreme laterial loads and shock...

thanks again

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I dont know, you might be better off using those taper'd bearing then...
As far as the length, its hard to say, are these factory length axles? I would say start kinda long maybe a few inches, adjust the axles all the way out. Make sure the axle will fully seat etc.Trim it down with a cut off wheel and grinder. Its much better to take out length than to add.
The factory buttons is maybe one inch wide or so.
Let me know if you have trouble finding a button, a old axle would work great, be the correct size, and much harder than a piece of mild steel round stock...
I might have a small chunk if you need. PM me later, I might be able to help a guy out.