Chryco had gone all in on the '100 million dollar look" for '55-'56 and I doubt the burrs were worn off most of the dies before the even more famous Forward Look debuted in '57.
Bet it would have been hard to throw away a press line for the sedan large pillar tool to update to the more thrusting rocket ship style.
They were definitely not throwing away any proven components such as the new-for-57 front end.
YT has a great video (lengthy) on the unibody conversion by Chryco and all the background on the departure from conventional body on frame. Cool pics/video of the clear lexan 1/4 scale model getting twisted and turned under black light (?)
For all the fol de rol, about the conversion, it's probably safe to assume there wasn't a lot of extra reinforcement needed for the Unibody. The look was so radical, that was probably the last thing on any buyer's mind.