You really need to read more stuff,
The current bodyshape commodore ss's have LS1's. The one in the darius video is a supercharged 383ci i think.
They engine before the LS1 was the 308 which is a 5.0L and i've read they arent that good.
You can't use the JTR mounting kit because that is for the older style engines. You need to use Johns Cars LS1 mounting kit and the headers from JTR and i think they sell them on the johns website. And you can use the 6 speed manual that can come with the engine but i read the shifter wont fit into the original hole where the Datsun one is/was. You will also have to get rid of the wings in the transmission tunnel and drill holes to mount the custom crossmember which you can also buy from that website.
The 6 speed manuals only came with the 5.7L holdens and is known as the MM6 six-speed Borg Warner T56 manual.
The later model VT commodores came with the LS1 but only have 220kw at the fly. That was the first commodore to come with the LS1.
The current model SS has 250kw but thats only with the SV8, SS and the Ute.
I can't answer your question about the front cut but call up some used parts places that specialize in holden.
Check out www.commodores-r-us.com
Read up from VT-VY.
I havent done this swap just read about it a bit, so i might be able to answer some more questions