The difference between 360KB and 500KB is not entirely due to space between sectors. A lot of the difference is due to clock sync bits, sector identification markers, checksum/ECC bits, etc. Keep this in mind otherwise comparisons are meaningless.
Taking the DEC RP03 disk pack drive as an example of an "early hard disk", I took a look at the maintenance manual for the associated RP11 controller:
RP11-C disk pack drive controller maintenance manual
The RP03 drive uses RP02P disk packs (actually the same packs as the IBM 2316), and these packs are 'hard-sectored' via a metal disc with 20 sector notches (plus 1 index notch). Rather than having 20 small sectors, the RP03 ignores every other notch to end up with 10 sectors per track, allowing each sector to be twice as big. These packs are spun at 2400 rpm, which works out to 40 revolutions per second, or 25 milliseconds per revolution. Dividing by 10 sectors gives 2.5 milliseconds per sector, or 2500 microseconds.
Page 4-24 of the maintenance manual indicates that a continuous stream of 0 bits would be written as a 2.5 MHz signal while a continuous stream of 1 bits would be written as a 5.0 MHz signal. This means that the 'bit clock' is 2.5 MHz, or 0.4 microseconds per bit.
Page 3-10 reports that each sector consists of 165 37-bit words, which is 6105 bits. At 0.4 microseconds per bit, that's 2442 microseconds per sector. Note that the actual data capacity of each sector is 128 36-bit words (4608 bits) so a significant fraction of the theoretical capacity is occupied by parity/checksum bits, clock sync bits and other housekeeping (1497 bits out of 6105, or 598.8 microseconds).
2500 microseconds minus 2442 microseconds leaves 58 microseconds of spacing between each sector, or 2.32% of the track (58 divided by 2500).
This is a very small amount of overhead compared to the 598.8 microseconds used for error checking and other overhead (24%).