the fins on the 1518 get in the way of the flux motor mounts. also, the xl plates you are getting will not fit the flux motor mount anyway. you can get a kershaw designs motor mount to go with the new motor and TVPs. it's only $20.
http://kershawdesigns.com
edit: on the homepage, click on the motor mount link and get the motor mount for the x/xl chassis that fits 540/550 sized motors. the castle motors are made to mount like 540 sized motors.
They say a good mechanic only needs 2 tools - WD40 & Duct tape. If it moves, and its not supposed to, duct tape. If it doesn't move, and its supposed to, WD40.