Thank you, that's invaluable information right there, 93x64!

That sounds actually pretty promising - as far as I know the trigger on the 550 is installed on the action instead the bottom metal like on the ZKK - that's not an issue? I thought if anything needs to be done it would be relief work in the trigger area because of that major difference.

With the spacer, do you mean that little round collet for the front action screw?