There's no reason the bumper shouldn't line up unless something is bent. The frame rails on these cars had the same dimensions for the entire run, the bumper mounts did change when front switched over to shock mount bumpers. But that's pretty obvious, there's only one mounting bolt for those and it's not anywhere near the right spot. So if you used the '68 brackets and had mounting holes, the bumper should line up fine when properly adjusted.
The fenders have the same overall dimensions, there's no reason the gaps should be off further than basic adjustments. On that note, the factory did used spacers on some the fender mounts to get the gaps lined up, it's pretty common to find the 1/8" factory spacers in several locations or even stacked on occasion. There definitely shouldn't be any welding on the fenders required to adjust gaps (swapping marker lights to match is a different story). There are several members on this board that have successfully used '74 fenders on their 67-69 cars.
The hood has the same dimensions, so if there's a 1/2" gap anywhere you still have alignment work to do.
The basic chassis from the firewall forward is dimensionally the same, so, nothing should be harder to line up after swapping the front bodywork with a different model year. There may be a few mounting hole location differences, and as you pointed out some of the wire harness pigtails may need to be lengthened or modified because of minor changes in the wiring depending on what years you're swapping. But you shouldn't need a cutting torch, die grinder, or welder to make anything fit unless you're swapping a non-shock mount bumper onto a car that came with shock mounts.
Basic panel alignment is all it takes.
