MOST of the body lengths, at that point in time, were due to the manner in which the OEMs designed their impact absorbing safety bumper systems. Some of those lengths could vary, too, with the addition of deletion of the vinyl/rubber impact strips, too, and/or the ridges which surrounded these strips, too. Can't forget the strips on the bumper guards, either.
The wheelbase differences in the same formal platform are interesting! To verify them, you'd need to also get into the FSMs and check the length dimensions of the rear leaf springs, which might be easier to change rather than hard sheetmetal in the platforms. THEN, such minor differences might be "measuring-related" than anything else.
There is also another little trick, used on the mid-60s B-body Dodges and Plymouths. Same platform and underbody, but the Coronet had a 1" longer wheelbase than the lower-level Plymouth. Same rear springs. How'd they do it? With a 1" "spacer" to mount the front spring eye's bracket to. Clean, easy, and inexpensive, while still remaining the longer wheelbase of the higher-level carline.
On the C-body side of things, there were some years when the wheelbase was stated to be 123.5 and the next year it was 124". Same cars, just one model year apart. What changed? "Rounding error", it seems. Rounded up to 124" for marketing purposes, as some claimed. Which is not that unusual in engine sizing. So that one OEM's engine size is not the same as a rival's engine size. Like the Pontiac 301 not measuring that different from a Ford 302, for example, as the Pontiac was a 4.9L and the Ford was a 5.0L. As I recall, there are a few others like that back then.
In ANY advertising situations, "more is better" and equals "superiority" of sorts. Not unlike in the 1990s when GM was touting their 4-cyl engines as being better than Hondas, in the GM literature. A 2.4L vs a 2.3L, "Advantage Oldsmobile". 3 more horsepower from the same smaller engine, "Advantage Honda". 1 more EPA MPG, "Advantage Oldsmobile". You get the picture. There were no measures of refinement, though. But when the WOT performance was considered, it was not "Advantage Oldsmobile", though. The ONLY way to determine what was really happening would have been to put the two engines in front of the SAME driveaxle/gearing in a common body with the same tire sizes. Then see what happened.
About a decade ago, some Mustang owners proved just how their minds worked. Seems that Ford was advertising that their 5.0L V-8s were actually producing about 7 horsepower less, so these owners filed suit against Ford for "false advertising". The factory documents were produced and the owners won the litigation. For what real gain? Bragging rights more than anything else, it seems to me. But also tends to prove just how much "5.0L owners" (the real ones) have a mindset that they have the fastest vehicles on the road AND will prove it when challenged, even covertly challenged (as I did a few times, even with my anemic '80 Chrysler Newport!). All it took was to just leave a red light before them! As soon as they realized that somebody was in front of them, their rpms would come up and away they'd go, having been caught off-guard. Such great fun that was!
And then there was the time I did a higher-speed challenge with my then 234K mile '77
Camaro 305. Just a slight cam upgrade (210 @ .050"/.440" lift) and a 600cfm 4160, but with the stock exhaust manifolds and the dual-outlet OEM Z-28 exhaust and 15" Z-28 steel wheels AND a 2.56 PTrac. I had covertly shadowed him in traffic, a "paper tag new" '87 Mustang GT. When we got to the top of a hill, I knew what was on the other side and there was not a vehicle ahead of us. At that time, he was in front and I was behind him in the center lane. At 70mph (55mph speed limit back then), he made his move and I followed, flooring the accel pedal for a downshift to 2nd, followed by a manual shift to 2nd, as the normal upshift speed was 75mph. Shifted to 3rd at 100mph. He suddenly backed-out of the throttle at 110mph. He had not gained an inch on me and my aged look-alike Z/28 Type LT. I eased on by him, not desiring to back out of the throttle that quickly. He did NOT smile and wage, like a good competitor might, but stared straight ahead gripping the steering wheel. I smiled and laughed. After I'd slowed down to legal speeds, he eased past me. Still looking straight ahead. I smiled and let him go on to whatever else he was doing.
Obviously, the truckers on their CB radios were talking about this deal. Maybe even some friendly bets? Don't care. Just proved how good an older, cast iron exhaust manifolded SBC might act against a higher-tech new Mustang 5.0L GT. I was satisfied. Obviouisly, though, I would have had no chance against him at the drag strip, so I took the race to speed ranges where I had some possibilities. Was I positioned to draft him only, not having enough more power to pass? Wouldn't that have been embarassing to him, getting passed under WOT? Have to choose your fights . . . and he probably thought he was racing a 350, rather than a mere 305, too. hehe Fun while it lasted, me not knowing the 234K 305 would even hold together for that kind of a deal. As it later went on to rack-up 600+ miles before we replaced it with the same engine specs, just in a SBC 355.
Sorry for the diversion. Just wanted to indicate how much some people tend to be worried about "little things" which really amount to even smaller things when they get to the rear wheels. That 7 horsepower at the flywheel, reduced by about 20% at the rear wheels, would need to be measured in fractions of a second at the drag strip . . . where driver technique, chassis set-up, and tire choice could over-shadow any results to prove their point, by observation.
Again, sorry for the diversion. Enjoy SAFELY!
CBODY67