Well this was back in 1.6.4, but basically I went and grabbed a version of the API from the latest 1.4.7 IC2 build. Nuclear Control generally used most of the API functions from the old 1.4.7 IC2 [albeit a few renames because of API changes]. Some other mods are a lot harder to backport considering they use API functions which did not exist back in the 1.4.7 version.
At the moment I'm attempting to edit Nuclear Control 2 [the re-code by xbony2] to add support for IC2 Classic. This may or may not be released publicly. It mostly depends on the Nuclear Control 2 license.
The main difference of my approach to this is that I used a 1.6.4 version of Nuclear Control for this addon instead of a 1.4.7 version. This worked very smoothly surprisingly.
I wasn't able to test this in my development environment due to there being no deobfuscated version of IC2 Classic, but I may make a deobfuscated version for usage [probably privately].
But coding for IC2 Classic addons is pretty much like coding for a IC2 1.4.7 addon. Hope this helped somebody!
Edit: In a bit I can provide a download for the API.