I supplied a kit that someone had an installer fit - the installer completely screwed the wiring up, chopped it all short then miswired it. I had to take it back and reconstruct the loom. This installer advertized as a Webasto heater installer!
Why you would need to change the length is strange, just coil any extra up. If you really have to shorten a loom don't start talking about cutting pieces out and re-joining, fit the correct connections at the end. Some of the joins that I have seen on looms have been appalling - those ridiculous plastic covered crimps are a great source of problems - they are usually poorly made and can pull apart and are a great source of corrosion both leading to voltage drop. ( The voltage drop on a long loom as supplied is negligible, people create more voltage drop by running the power cables to various central systems with extra fuses and connections rather than wiring direct to the battery).