In our promaster (ducato) we have about 4 inches extra on either side and iI can stretch out just fine. That being said I’m just a tad over 5’8". What I see people do often on these vans is box that section out and just use some thin insulation in those areas. I think the interior there is 77" and I remember when buying it was the only one that could fit a full size bed sideways, but I’ve never tested that theory myself.
The flares seem crazy expensive to me and I imagine by the time you get them installed and painted to match the van, you’d be spending well over $5k or morei. Not something I personally would want to attempt to DIY. I would imagine there are lots of issues there with possible water intrusion if not done properly.