My understanding is Speartech builds their own harness and does their own ecm programming. I've exchanged emails with them a couple times on building an E67 based setup for my build. They specialize more in stock or near stock configurations than the high performance market.
FWIW - I have no affilliation with them.
From their website
"We have been building standalone wiring harnesses for the Gen 4 (58x) LS platform of engines since their inception.
We developed the first LS standalone wiring harness around 1996/1997 and have been building them in-house for thousands of automotive enthusiasts and professional companies around the world since then.
We build our LS swap harnesses on one of the many different live engine and transmission setups we have in-house which closely reflects the swap you are performing in your project at home or in the shop!
It is because of this process, our harnesses are the best running, looking, and fitting available.
The other major benefit of having us build your LS swap harness on one of our live engine and transmission setups is that we actually test run each and every harness we build prior to shipping. While your harness is running the engine, we have OEM GM diagnostic hardware and software hooked up to verify our work, and to help ensure trouble free operation once you receive the harness.
Once the LS swap harness has passed all of our diagnostic run tests, we cover it with our exclusive, heat resistant, woven fabric covering. This isn't a cheap see through plastic mesh covering, this is a nice quality covering and offers the correct fitting finish to your new Speartech harness that our customers expect.
The process and practices we choose to use takes a little more time, but we feel are necessary steps in order to ensure the end user has a good experience and gets something that "just works", and with proper installation, we know it will - we have already tested it on a live engine and transmission!
Put 23 years of GM Engineering experience - and now over 20 years spent perfecting the LS swap harness manufacturing process - to work for you!"