I reckon they would have been best off stretching a 90 chassis, since it's a bit lighter... but either way, I suspect they wanted to avoid a cut & shut job on the chassis, since that is a bit of a job to do well, and keep it strong.
The RR chassis is designed to run at 100" with a good amount of strength, and welding outriggers, etc. on is a lower-risk way to go in many respects...