Hideeho
When I 1st went to the shorty I use 2 medium pads & a BUNCH of thin pads so I could play around with exactly where I wanted the lipo to sit. Once I found where I wanted it, I cut an sc10 pad in 2. This fits in, but doesn't hold a shorty tight. To tighten everything in the tray up I add in a thin pad. This also gives the benefit of an easy way to adjust the weight balance at a moments notice by moving the thin pad to the back or front. I run a t4 chassis, so the sc10 pad cut in half will be 7mm to long to be able to use the small pad the way I do. If you want to, you can cut a small part (7mm) off one of the pads to fit in the thin pad.

Where you end up setting the shorty is a matter of taste. If you have your electronics in the side pods (standard config), you will probably end up with the back of the shorty around the front of the "U" brace (I set mine about 5mm forward, t4 chassis ). If you end up with the electronics in line, you will probably end up with the lipo all the way, or close to it, forward (I put 1 thin pad in front).

Going to a shorty can be a good move if you are looking for a quicker responding car. If you are on low traction it can be a handful. My local track I would describe as low to medium traction. Several guys tried it & they all went back to stick packs because it was too twitchy. I stuck with it because of the t4 chassis. The longer chassis slowes everything down & makes it more stable. The lighter shorty pack adds back in some edgyness.