I emailed Performance Specialties this AM and received this prompt reply:

"For an installation on the 46, the corners of the rear piston skirt need to be trimmed to clear the mount. TT 46 used to fit without modification, but they made a change somewhere along the line and the skirt measurement changed. If you'd like us to make the mod, send the engine and we'll be happy to do it for you at no charge.

Dave"

Mystery solved! Top notch service from a top notch company! Thank you, Dave!