I am really surprised this didn't kick up

An A body is built the same way as a B body.
So stretching the B body car doesn't allow "too much" flex in a B body between the front and rear then?
If a car has frame connectors there is more holding the front and rear together than just the rocker panels.
Would I go to the trouble of adding them to my daily A body?
Probably not just because of down time and work.
But if I've already got them in a B body, I'm leaving them in.
The car is almost 50 years old.
It's flexed enough over the years already.
That's the way I'm going to look at it.