Subframe connectors

MUST is way too strong of a word here. CAN or MAY are probably more appropriate. I've done it both ways and it worked out great for both. Doesn't matter how it's done. As long as the doors shut well and the gaps are good, weld 'em up.
So you are recommending to literally stiffen a bow into the car? You "May" have done it however saying as long as the doors shut its ok.......... that sounds like a fairly agricultural build theory. Not sure why you would risk it given how simple it is to load the wheels up. Big difference between a door closing and a door with a perfect panel gap.