Frankenmech rules starting page 189 Strategic Operations
Engine restrictions as found on Page 215 of Tech Manual
Additional relevant rulings
here and
here.
Keeping the above rulings in mind I find myself wondering if in the case of using an Industrialmech side torso but a Battlemech center torso that uses an XL engine can still be made to work despite Industrialmechs not being able to mount XL engines.
Rephrasing my ramble there as a question:
Can you use an Industrialmech side torso with the center torso of a Battlemech that has an XL engine?