I just got around to reading the August 2022 issue of Mopar Action magazine. (Yea, I'm a bit slow!) In one of the articles, written by Richard Ehrenberg, it is mentioned that the Mopar A body was the only car ever certified to run on the NASCAR superspeedways without a roll cage. (that statement is on page 66) After doing some internet research, it appears the NASCAR compact race series was a 1960 and 1961 only deal.

The early 60's A bodies looked considerably different than the later A bodies. Visual differences aside, were all of the A bodies the same structurally? How did the strength of the later A bodies compare to the 1960 and 61 models?

Those were the first unibody cars that Chrysler made, that ended in 1962 when they switch the B bodies to unibody construction: down: No more full frame b bodies after that: whiney:

Without looking up "Plymouth Valiant" on Wikipedia or looking at Mymopar, etc, as I recall the 60-62 A-body cars had unique structural stampings; obviously the bodies were unique.
Mom had a '60 Valiant that I drove daily, and a girlfriend had a 64 Valiant, and though they drove & handled similarly, I don't think much was the same.
The Slant 6, maybe some mechanical & suspension items, and maybe some electrical stuff likely transferred.
The 1st-gens dominated that NASCAR series, and IIRC Mopar advertised the '63 (2nd-gen) as being all-new and tested for a million miles.
I've enjoyed driving every generation of A-body.

The 61 & 62 A body may have had a slightly better construction, but I suspect the difference was more likely NASCAR's safety requirements changed a lot between the late 50s and the early 60s, and has been growing more concerned about driver safety ever since.
Early auto racing was a deadly sport. It was because of all those early deaths the rules began to place more concern on the wrecks being more driver survivable. Better built cars, and better race track safety were required to keep racing, and new rules forced those changes.

Since Chrysler was the only company that really took the sub compact series seriously, I would guess no roll bar was more a negotiated point than anything.