I did a cover off on the Euro 420 engine that was put in my Euro 500SE. And I'm sure glad I did as the bolt that hold the largest of the bottom rails backed out till stopped by the cover allowing the rail to sit at a slight angle ,it was paper thin with very deep grooves.I also replaced the oil pump chain while cover was off.

I used a large screw driver and a pair of needle nose vise grips to hold the screw driver that was wedged in one of the holes on the bottom of the bell housing catching a tooth on the ring gear,then put a long steel pipe on a breaker bar with a new 6 point 27 mm socket and pulled hard.
Yes you drop the upper pan on it's bolts to give a little clearance at the heads, and both cams should be on their marks when at TDC on the compression stroke.I cycled the engine many times by hand double checking before I started it, if you take the oil pump out you have to fill it with oil before reinstalling.I highly suggest you have a manual that shows the cam marks etc, I was a fool and did the job with out one a friend gave me the torque specs and away I went.If the engine last another hundred K it will be easier the second time...........
William Rogers.......