I believe the tube is irrelevant, and all you really need is the finial. Amazingly, out of the (literally, I just counted) 42 pens on my desk, I don't have an Atrax with me, but I just compared the Triton I have to the Jrs (Gent & Retro) pens and the assembled Triton body is significantly shorter than the others. It is significantly shorter than even the Baron/Navigators. Both the post and the nib assemblies are a bit longer on the Triton, but the difference in the tube lengths is greater than the difference in the hardware.
If you are only replacing the non-postable Atrax part with the post from the Triton (or Jr. Series, as that would work also) I would think the difference in the size between the two should easily be handled by the stock spring that came with either post end, but either stretching it out a bit or getting the longer springs that Exotics sells wouldn't be a bad thing...