There are different schools of thought on this. On one side, you have the old school guys like Uncle Tony or the local restoration shop who bent up my exhaust system who don't think you need chassis reinforcement unless you have a Hemi and a manual transmission with slicks. On the other side, you have the protouring guys who don't even think the chassis nor even the torsion bar suspension is acceptable at all...I think either extreme is probably objectively wrong.
These cars were mass produced and built to a minimum standard to handle road conditions, powertrains and expected longevity of their era. A typical car from 1964 to 1976 would be long off the road before 120k miles. IMO, it's important to keep that in mind and to base some forgiveness on that. So when you see that the font and rear subframes are only connected by the rocker panels and floor boards, it makes sense. The body flex was acceptable for the time, would last still longer than the expected lifespan of the car and IIRC it was possibly even desired as it supposedly improved ride quality. Regardless, this design worked when you ran on 14x5" bias ply tires, had a 145 hp slant six running through a gentle shifting automatic.
It's also worth noting that convertibles and performance variants had (or generally had) torque boxes to reinforce the structure while basic transportation appliance versions did not. Obviously, it was recognized back then that the basic A Body platform could benefit from strengthening.
These cars weren't supposed to last this long. The engineers of the Abody architecture would never have imagined that we would sitting in front of personal computers debating the need to reinforce their chassis deign more than half a century into the future...never mind doing this on a tiny rectangle computer while we sit on the toilet.
IMO, what also makes this debate difficult is that these cars have a minimum of 44 years and God only knows how many miles at this point...there's wild variation in the results of these years and mileage depending on where they were kept, if/how they were looked after, how they were driven, etc. Second, there were variations in how well built these cars were from the factory. One car would often have better welds than the next within the same run.
Plus, there are at least two different wheelbases for the A-body, and year to year changes. There were also convertibles, coupes, sedans, two door post and wagons each of which puts different stresses on their respective chassis.
Finally, there are different standards as to what is stiff/strong enough. If your only goal is to "not break your windshield while running a 440," subframe connectors would probably do the trick. If you want to preserve chasis rigidity to improve handling and responsiveness, you'll proably also want torque boxes, inner fender braces, K Member reinforcement/improved welds, steering box mount reinforcements, radiator support, etc.
For the sake of the OP: at a minimum, Assuming all else is in working order, I would add factory style torque boxes to a big block swap car. The original engineers even recognized how important they are.
Personally, I want all of the strength that I can get so I can just enjoy the car without worrying about stressing the chassis. Plus, I want all of my cars to handle predictably and feel good going around a corner, even if they're not sucking the carbon fiber off of McLarens. With my Dart, started with suspension changes and immediately noticed that after adding stiff shocks, torsion bars, heavy duty leaf springs, sway bars, a quicker ratio steering box, etc, the flex in the chassis became more obvious and made the car feel sloppier than I liked.
So I added subframe connectors and torque boxes and the difference was immediately obvious. The car is much less "jiggly" and it just feels a bit more solid all around. Is it "stiff"? no... but I cannot imagine driving one of these cars aggressively on modern radials without reinforcement, big block or no.
False.
General Motors A Body platform cars are body-on-frame.
