Justin...
I did my entire swap by myself in my garage. I extended my harness 16" (3 feet is WAY too long...i mean WAY to long) Mine was a PERFECT fit. 18" is the longest I would tell anyone to do theirs. The second is that yes, you need the subharness. See, the ECU plugs on the 1jz harness will obviously fit into the 1jz ECU. The problem lies with the other 2 or three plugs on the 1jz harness that are near the ECU harness. These plugs (different ones are found on the 7M harness in the same location) will not fit into the connectors that are under the dash that the 7M harness plugs used to connect to. I would NOT recomend that you cut ANYTHING from your 1jz harness...the only cutting you need to do is the extension. BTW, there are about 110 wire in the harness....I separated all 110 into 3 groups of wires, and I did one group at a time...and one wire at a time. Some of the wires (O2, crank angle sensor, knock sensors, etc) are shielded cable. You will want to extend these using 2.0 USB cable. Get 2 10 foot sections from ebay for about 5 bucks...don't buy 6ft for 35 at radio shack.
Trev