I need a small master cylinder with 7/8" bore

Not sure what to say here, other than your parts selection is very questionable.
On top of that, not sure why you would mess with the original brake rod, when it should be a bolt in process.
I get it, hot rodding is cool. But when it comes to stopping, I like to keep it real.

I would have gone with a rear wheel drive aluminum MC from a Dakota.
Used the stock, unaltered brake rod, and bolted it all in.
Plus, it is imperative that you use the small rubber brake rod bushing, that some mentioned.
It does a couple things. It helps secure the rod into the MC.
And it keeps the rod positioned in the MC, where it should be. Also keeps it from rattling.

Hope this helps.