E30 Zone

i am in the process of removing the heater box.

it is disconnected except i cant access the A/C lines to the expansion valve as there is too much stuff in the way.

what is the best approach here?

1) leave the lines attached to expansion valve and pull box with lines throught the firewall grommet after disconnecting at engine bay?
2) or is there a trick to access the lines at the expansion valve?

Its common to pull the heater box out with the 2 lines attached, however, I have changed the expansion valve with the box in place, cant remember exactly how I did it, but it was not fun!

Assuming you're RHD and you've removed the dash?

Yep it's possible to disconnect the A/C lines at the expansion valve - but as Dan mentioned it's possible but awkward with restricted space.
If you're going to remove the box with the A/C lines attached you'll need to move a fair bit of stuff to access the firewall grommet for the A/C lines, especially to correctly seat the grommet. e.g. engine ECU

If you have the dash removed, I'd say it's easier to disconnect the at the expansion valve.
if not, you'll need to decide if it's worth spending some time in a cramped space to disconnect the lines vs. pulling a bunch of other items out to get to the grommet.

Also if you're based in OZ and you're going to re-charge the A/C system with R134a better off swapping out the old flexible lines for barrier hose that's compatible with R134a.
Don't want to go to all this effort and have to be topping up the system because of the older R12 hoses that don't hold R134a as well.

i managed to get the lines off while in place wasn't much fun though. i didn't want to pull it out with lines attached as i have extra wires and crap under there due to aftermarket ecu and previous owner crap stereo system and was bound to get tangled

For install i might install hand tighten, then sit it in place a bit away from fire wall to give better access so i can clock them suitably

Is it best to install the AC line grommet to the firewall first and then feed through lines? or feed lines through grommet on the bench and seat grommet?

Same question regarding firewall grommet for cooling lines, preinstall grommet on the two tubes or fully install in firewall and slide tubes through as box is put in place?