although it appears entirely possible to use your chassis to swap to the v8 motor,,,,you'll most likely need tons more parts off of the 420 chassis to gain a proper car ie> upgraded motor mounts, upgraded front suspension/chassis and brakes/steering, bushings, etc.....to handle the extra weight of the V8 (as engineered by MB in the 420 chassis)
perhaps the most economical solution is to seek out a used 380 motor from one of the reputable MB boneyards (it would be the least labor intensive exercise to get you quickly back on the road)- - I'd search for one w/ a compression and leak down test and w/ a guarantee
another option is to seek out another vehicle w/ the timing chains done (w/ records) and go from there....complete running 380's appear to be very affordable and perhaps more economical to acquire vs. repairing the current vehicle?
in either case, rebuilt MB motors are not cheap, reputable re-builders do exist and offer decent warranties (if you like your particular car enough- it maybe worth it to you to fix it)
hope this helps