I cut mine out heaps... cut it with a hacksaw to keep the slots as small as possible. It doesn't seem to have moved yet. I will re-measure and double check when i weld back in. For the mean time though, with all the trial fits etc it sure is a lot easier to slide it in and out this way.
It would be easy if you had a hoist... just make up a frame or something to sit the motor on, lift the car up, wheel the motor under, and drop the car on top.
Think i'm gonna get me a hoist....