Tikka magazines are all one length and use one bottom metal. XLR chassis are not for Tikka magazines but rather AI pattern mags which differ notably for long and short action.
The Tikka long action chassis accepts the 3.560" aics magazine. This is for cartridges like the 300 WM, 7mm Rem Mag, 7mm PRC, etc.
The Tikka short action chassis accepts short action AICS magazines which are for cartridges like 6.5 creedmoor, 308, etc.
The action cut for both chassis is exactly the same since all tikka actions are the same, only the bolt stop changes from a long and short action to dictate the amount of bolt throw you have on the action. Let us know if there are any further questions!