I've actually got Bigsteves car in my shop ATM and have just fitted some in old school speak "Fast road cams". With the exception of the exhaust manifolds and the Magnaflows on the back of it the rest of the system really needs to go by way of the bin as I'm pretty sure that my 4yo son could make a better one (no flanges and leaky welds). The man to talk to here in Perth is on leave ATM and away for another 2 weeks (he does all our custom Hilux extractors) so if you wanted to go with fitting extractors a "Tri Y" design would be best with hi-flow cats given your use of the vehicle with an "X" pipe rather than the balance pipe that is fitted (those went out with VL Commodores) running 57mm tubing to a pair of resonators then 63 to the magnaflows should do the job nicely. As she sits ATM wait until you hear it with the new cams. It makes a worked LS1 sound like something that Granny drives to the shops on pension day
PLEASE do not take what I am about to say as an insult because I have great respect for individuals such as yourself that are building these or any engines to higher output.
Your exhaust recommendation above is way better than most but I respond to give further insight for readers here.
The stock Y pipe (which Big Steve does not have) is a very good part. MUCH better than what most think. It has twin 50mm tubes (1.97") feeding a single 60mm center tube (2.36") using a very effective collector design.
I built a very nice 57mm (2.25") into 63mm (2.50") merge collector (see image below) and donated it to the very first S&S header install on a GS400. A shop built mandrel bend 2.25" tubes off of the catalysts outlets into this merge collector. The peak power changed by ONE horsepower but the engine became mushy and lost response.
The Y pipe above was a direct stock bolt in replacement so the owner was able to swap back a stock Y pipe and all the response and low end came right back. He was also able to run a GS430 Y pipe which has identical dimensions except has a catalyst in the 60mm section instead of the resonator in the GS400 pipe. Power and response identical between the two stock pipes.
Another S&S header user built a 2.50" into 3.00" Y pipe using a quality merge collector from Dr.Gas. His car seriously lost low end response and power. After discussion with me he had a 2.00" into 2.50" pipe constructed and was AMAZED at how much low end came back and even felt the top end was stronger (subjective as no dyno was done).
These events occured about 6 years ago and made me reconsider all my thoughs and now recommend the STOCK GS400 size Ypipe for all motors up to 275rwhp (205rwkW). SC400 and LS400 Y pipes neck down to about 55mm in the center section so those are more like 250rwhp pipes. For power from 275rwhp to 300rwhp (or if you have an SC or LS) simply change the 60mm section for a 63mm section (2.36" to 2.50") ... AFTER THE MERGE... DO NOT CUT OFF THE MERGE... which gains nearly 13% in flow area (using .065" wall thickness tubing, standard 16 gauge). You can use either open tubing or use a perforated resonator to reduce sound. Do NOT use a resonator that has less than a TRUE 2.50" center with SMOOTH flow. Some cheap ones have louvers punched into the flow area reducing flow capacity substantially and would be worse than stock. Go with a quality part like Magnaflow or Dynomax.
It is good to consider that the flow area of a 16 gauge 50mm pipe is 2.65 sq/in... so two of them have 5.3sq/in of flow area. The stock 60mm has only 3.9sq/in so the two smaller pipes have more total area than the center pipe making the center pipe the restriction. That said some 400rwhp supercharged GS4xx cars have ran that stock pipe. Increasing the center to 2.50" gives you 4.4sq/in of flow area STILL less than the twin pipes feeding it.
Now, I am not saying the stock pipe cannot be improved upon BUT I am saying it is harder than most believe.
FURTHER complicating the above is the RATIO of the Y pipe. The stock pipe has 5.3sq/in accelerating into 3.9sq/in for a ratio. Now MANY things come into play other than strickly area but this is a good way to visualize things. The stock twin tubes have 26% more area than the stock single tube.
If you look at the same for a 2.25" into 2.50" pipe the twins are 38% larger. This requires more energy to accelerate the gas than the stock sizing. If you want to use 2.25" tubes you need a 2.75" single to have a similar to stock 24% ratio. You DEFINITELY want acceleration in the Y section as this creates scavenging but too much creates unnecessary back pressure and reversion at lower RPMs and flow.
NOW, since the single tube has less area than the twin section you MUST decide when you need more than 60mm, then 2.50" before upping that size to 2.75" (70mm). Consider how much power a 3.00" (76mm) pipe can support on a turbo engine like the 2JZ-GTE which is upwards of 700rwhp... so how much do you think a 2.50 can support? Well over half for sure. 2.50" STRAIGHT pipe can support 350rwhp. So, before increasing the size on the single section consider how much you NEED.
I have had many PM me requesting that I build proper replacement Y pipes. I have been close to doing so on a number of occasions and I am once again getting close to doing so.
Hope this helps.