I think we got the .620 id fire sleeve. As long as you didn't have to push/pull it over a 120/180 degree fitting you should be to get it over the -6 ends fairly easy. With a little more effort I'd bet it would go around a 120. I'd probably stick a lil piece of rag or something in the fitting to help keep the fibers from possibly ending up in it as you push/pull it through.
I'm using 2 cannon plugs/bulkhead connectors(whatever ya want to call them) to break out the ecu to "sub harnesses" through the firewall for the motor, 2 31 pin deutsch autosport connectors,essentially one for the top of the motor/sensors and another for the coils/lower sensors. A 3rd 16 pin will carry the larger draw stuff into and out of the sealed bussman relay box, then a mix of weatherpak and deutsch dt connectors will make up the rest of the wiring in the buggy, lights/gauges/fuelpump/fans and so on.
You can get pretty much everything you would need at prowireusa.com,techflex,wire,loom and all. Kinda have to know what your looking for, as you have to buy single pieces rather then a "kit"
https://www.prowireusa.com/c-6-deutsch-autosport.aspx?pagenum=1
I tend to use amazon because...well it's easy.
16 pin
https://www.amazon.com/Deutsch-Bulkhead-Connector-12-16-20-Contacts/dp/B01H8H27A8
31 pin
https://www.amazon.com/Deutsch-Bulkhead-31-pin-Connector-Contacts/dp/B07CWQZ6WH
Techflex, you can shop around and find it in just about any size you need, it's fairly easy to roll up tighter, and it's got enough overlap that you can usually squeeze a little more wire in it then the advertised diameter and it still wraps up.
https://www.amazon.com/Techflex-Split-Braided-Cable-Sleeving/dp/B01MXMAQYC
I decided against the full blown reychem shrink wrap/shrink boot/joints on the harness, mainly for time and it's a bit over kill for a buggy in my opinion unless you just like fancy wiring. I use some adhesive lined heat shrink to hold the end of the tech flex, and a wrap or 2 of tape at any of the splits. I'll see if I can dig up a few pics from the last harness.