Well in the height of the 'Grey Market' era in the mid 1980s, MANY 380SE/L and 500SE/L were brought into the US. Even today I can search the want ads, at least in Texas, and find these vehicles if I search hard.
So I would just keep looking around and you will find a pair eventually. Or start sniffing around on ebay, someone is bound to sell some there and have no idea what the hell they have listed.
http://timevalve.com/cat/euro/p26b.html
I have all that stuff to install on my 560SEL. I picked up the downpipes in the junkyard years back. I just need to attain a high flow dual in/dual out cat with integral O2 bung and a few mandrel bends to connect the downpipes to the cat assy.
Without some research I cannot say if the blocks are the same, I seriously doubt it. To increase displacement that much, just flat out requires a taller deck with a larger bore. I remember the 5.0 and 5.6 are the same block for some reason.
Think of it this way... 302 Ford and 351W. Very similar but very different. Taller decks by over 1", larger headbolts, longer rods, larger rod bolts, totally different piston compression height, different oil pans, different intake manifolds, etc, etc, etc.
The stock mufflers flow well for what they are on these cars, these are not Volvos where the car cannot breathe. When the system fell apart on my 300SEL (PO and all their city driving), I cut open the stuff to have a look (it looked simply like a Magnaflow internally...). Just the catalyst assemblies flow like ass... just like the '80s BMWs.
The great news? You can scrap these old catalysts and make a bundle, I received right around $250 for the one off my 735i... and your 420SEL has two! One huge main cat and that precat that is the size of a normal unit.
Hope it helps and keep us posted.