The 1973/4 'flat top' SU carb cars had problems with vapor lock. The under hood heat actually boiled the fuel in the lines and turned it to vapor, making the cars impossible to re-start after they were turned off. We called it 'after boil' in the 1960s, but Datsun never alluded to that. The fuel pump and carbs are not able to push vapor, only liquid. The cars had to cool down awhile until the fuel changed state back to liquid. Then they would re-start, amid cheers from the Z crowd and the driver's impatient girlfriend.
Both Datsun and the individual car owners tried many little tricks (some of them pretty dumb) to combat this problem. I suspect this upsweep was an attempt to let vapors and bubbles rise to the top of the fuel rail, allowing the remaining fuel below it (btwn there and the carbs) to stay liquid. They also revamped the fuel return lines, which carried 'waiting' fuel back to the gas tank to be cooled before it vaporized.
The upsweep should have absolutely no effect on your engine operation.
Some of our 'street fixes' in the 1970s included wrapping the entire fuel line with aluminum foil (looked hideous), clamping wooden clothespins along the line to act as heatsinks, and one of my buddies actually rigged a coffee can which he made the fuel line run thru. He kept filling the can with crushed ice (a major chore) every time we stopped somewhere. We called it 'the still'. Two other guys rigged small electric fans aimed at the fuel line. We laughed then, but the 280ZX cars later did essentially that very same thing a decade later. The simplest thing to do was open the hood when you parked, to let the heat escape sooner.
Some of this stuff actually worked. Other ideas were downright goofy.
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Probably.