Broken rotor on crab distributor

[34pickup]

I was reading another thread about some people experiencing a broken rotor when converting over to a crab style distributor setup. Well, I ordered the crab conversion kit and had that happen too on my 1948. It came with the cap,matching rotor and clips. I put the distributor together and spun it around and it seemed to turn freely enough, no binding, felt like only the points were offering any resistance. I put it on the engine, it fired once and died. After a while I decided to pull the distributor back off and yep, the rotor was broken off.
So, now I need to buy another new rotor and hope the same thing doesn't happen again,they aren't cheap. I read where I think it was Bubba that files the rotor down, but how do you know how much is correct?
Doesn't anyone make a proper fitting rotor that works without breaking or having to be modified to work?