Just do a little filing, it's not that big a deal.

I finally got some 6.1 caster blocks and the steering knuckle is a real tight fit inside. Basically the caster block is not tall enough. It looks as if the ends are pinched together; you can see that the top and bottom are not parallel. I chose to modify the steering knuckle instead of the caster block because I figure the caster block is more likely to break and I don't want to have to modify them every time.

So a little hand-fitting is a pain in the ass, and holy heck are the new caster blocks sloppy. But that's what we get for not buying an Xray