is it better to use a shorty battery pack to race with ?
do you move it to ward the front or rear. carpet track stock motor?

I suppose it all depends on how you define "better". Using a shorty lipo should reduce the car's overall weight and another advantage is that the space not used by a standard lipo can then be used to place electronics like ESC or receiver to ensure everything is as low as possible in the chassis. The lower the better. You can also move the battery wherever you like, front to back, using spacers that can easily be made from simple Styrofoam blocks (or whatever) of various widths to tune how your car handles.

Yeah Racing sells an aluminum shorty battery pack kit in blue and red that uses a large O-ring to secure the battery that looks nice. I've often found that the aluminum bits are heavier than the stock plastic ones, but in this case you're likely OK. Another option I found was a seller on Etsy who has a 3D printed kit to accommodate a shorty pack. Looks nice and should be light weight too.

Might be worth a try.

I ran mine with a shorty pack in parking lot series and rally class on carpet and it drove well. But, my friend who is a far better driver than me runs a low profile full size pack on carpet.

That is the biggest question. It's all dependent on what you want from your TT02.
Some like the shorty for lighter weight, I personally like the stick pack from Protek 5000mah LiHV for better weight balance.
I tried a shorty, a heavier brick style lipo and my Protek and personally I feel the extra weight balances out the chassis for better left and right. Try each and see what your preference is.

Has to have wires and a plug to be legal everywhere Ive been.

I use the Protek Mudboss stick lipo.

My friend uses low profile Zee batteries.

I went through this with all variations of lighter and heavier shorties and I made different battery bars that would position them at various spots forward or rearward. I did this while trying to balance out the car perfectly on the scales depending on the shorty weight. The car drove well but then the one race I had forgotten to actually hit start to charge my battery so I threw in my full size Protek battery which made the car heavier and unbalanced on the scales but it drove so much better and that final came down to just a nose that night. I've kept racing with the full size pack ever since because it just feels better to drive. So I say if you have different batteries experiment and see how the car feels to you when driving when your tires are in a similar state.

For the TCS race at 180 Raceway I ran the Gens Ace Redline 2.0 6500mah battery and my car was the fastest GT ProSpec there, just not the driver...LOL . I qualified 3rd and was very close to the front two runners but screwed up in the main and fell back. But if you are running GT ProSpec and following the 1400g minimum weight rule then the way to go is a full size pack that gets the car to the minimum 1400g. The bigger packs have lower IR's and much less voltage drop off.

If you don't have a weight limit rule, then run the shorty pack to the rear. Before the implemented the 1400g minimum weight rule by GT ProSpec was under 1200g with the small shorty pack.

Can you tell me what nuts/bolts you used for the for the pass-through steering mod? Also, assume this just provides more strength than the 54550 TT02 Low friction Step screws?

Ah, ok... I see this is how the Type-SRX has it....

When threading the 3x23mm steering screws from the bottom of the chassis, it DOES help to apply some grease/lube on the screw threads. Pay attention when starting the screw into the hole that it isn't off center/at an angle.

This is a better option than using the screws that screw in from the top. The top down screws don't go all the way thru the chassis and have the potential to break the chassis/hole where the screws go in. This includes the small set screws that are included with Tamiya 54574 or 54752. In fact, I would highly recommend getting a new chassis if you are upgrading from the stock self tapping steering screws when going to 3mm machine thread steering screws.

Here is some part info on TT02 stock steering screws & the low friction option from Tamiya
Stock screw is a 3x18mm step screw - Tamiya 9805573 (https://www.tamiyausa.com/shop/7-dig...p-screw-44028/)
Low friction TT02 upgrade has a hex head - Tamiya 54550 (https://www.tamiyausa.com/shop/optio...ep-screw-2pcs/)

Here is the part number for the flanged tube - Tamiya 9803242 (https://www.tamiyausa.com/shop/7-dig...ed-tube-47439/)
**Note: I would either suggest purchasing one of the Tamiya TT02 Aluminum steering upgrades or aftermarket aluminum steering upgrades which will include the appropriate bushing/tubes as part of that hop-up.

On the diagram above using the 23mm screw it seemed easier doing it the opposite way since the bottom of the chassis has the cutout for the nut. So the screw and washer going from the top down into the nut in the recessed area on the bottom.

While you can use the 23mm screw from the top and the nut at the bottom of the chassis, it will make it more difficult to remove the steering arms. Because the screw is threaded into the plastic, you will have to totally back out the screw if you want to perform any maintenance on the steering arms/components. Having the nut on the top makes performing any maintenance on the steering simple/quick which is important when racing.

can you share item number for 39mm dcj? do you use stock front diff? what diff cup do you use?

You need 42363. It’s longer.