If the 420 has a similar system to the 560 then there are two squared rings that support the outer sides of the center mufflers. Those squared rings are a real PITA to install. You might find it easier to place a jack under the rear muffler, slip the O rings off the rear and lower the jack. Once the exhaust is lowered, get the squared rings on the body side of the hooks, then lift the exhaust. It may take another jack to carefully lift the system near the center mufflers to get the squared rings over the muffler hooks. Once the squared rings are in place, reinstal the rear muffler O rings.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.