In case anyone is trying to get to the proper gearing on this car for a USVTA car using a 25.5 it's not an easy process on this car. I figure I would save everyone else from making the same mistakes I did to get it to work.
First, if you want to use a 21.5 or 25.5 motor, you are going to need the vertical motor mount (SAK-66/PK). Don't even bother with the stock setup. I haven't found any small enough spur gears that had holes for the horizontal motor mount.
The 3racing website and a few people here expressed that it is mandatory that you use SAK-38/WO Soft upper deck with this, which is true, however, the car comes with the soft upper deck, so you do NOT need to buy another upper deck. I don't know why I assumed the "soft" upper deck was aftermarket and not stock, but I was wrong, and I ordered that part with the vertical motor mount. You don't need to at all, as it is the same one you already have. Unfortunately, I found out the hard way.
Anyway, I am used to having small spurs and large pinions for VTA spec racing, and the small spurs would not come close to the pinions when the motor was in the closest position to the spur. Everything I had in my box was either too large or too small to work. I wound up in a trial by error experimentation with expensive spur gears to get the right mesh and alignment.
So the absolute smallest 64p sour you should even try is an 83t gear. With that and a 25.5t spec motor, I can get a 40t pinion on and meshed properly to get a 4.040 FDR on the car with a 37t pulley on the diffs and a 19t pulley on the spur (not stock). I could probably get a 39t to work, but it would be REALLY close. Anything larger should work fine.
I would recommend at least a 85t spur for proper mesh and alignment on a spec car using anything 21.5 or 25.5. The 37t/19t pulley setup helps a lot in getting the FDR in the right window.
Hope this helps someone else avoid the expense and time wasted that finally got me to the right solution.