Tolerances can get a little squirrelly with 3D-printed parts. If the cross-bars are too tight, you can use a 3mm drillbit to clean-up the hole.
Glue the cross-bars into place and then glue the entire roll-cage into the body.
An M3 Tap works a treat if you have one...
Nice and slow - so that you don't crack the plastic.
Repeat the process for the other tail pipe!
Skip to the electronics step and get everything wired-up... then come back and use four short plastic screws to secure the chassis to the body.
Two of the screws go in mounting holes just behind the servos.
...and two near the axles.