Mercedes: `91 Mercedes 420 and 560SEL help, front end vibration, david chapman

Question
Hi,
I am a skilled mech in Phoenix but have very little exp with Mercedes. I have a `91 560 with 120K miles on it, car is dead sharp but has a front end vibration that is apparently engine related. Previous owner was told the front end needed work so had it completely rebuilt but vibration was still present. Took it to another shop that was a Mercedes specialist and was told that the engine was low on compression due to tapered cyls from running low octane gas. He got disgusted and sold the car. I have picked up a wrecked `91 560 SEL(115K miles) with a great engine and by happenstance a `91 430 SEL(155K miles) that is also dead sharp but with a blown timing chain. Turns out that my lady fell in love with the 420 (?) and wants me to fix that one. Not finding much for a reasonably priced good used engine for the 420 and I am concerned by the price of parts to fix it, so am thinking of just selling the nice 560 AS-IS and putting the 560 engine in the 420. They appear identical and I have the complete wrecked parts car so I can swap the engine and trans as a unit, computer etc if necessary. Is this a viable option, IE are they as compatable as they look? Would this morphadite the 420 beyond all sense if I ever wanted to sell it? Should I first tear down the 420 engine and see how bad it is? I have heard they are an interferance engine and the price of parts is astronomical. I can see this turning into a much bigger project that I first anticipated and am mulling over the best way to go. Any input would be greatly appreciated. Thanks David Chapman.

Answer
Answer:
If you morph it, that wrecks resale value for most people.

I would suggest tearing down the engine, you can simply call or e-mail Phil for parts assistance.
Toll Free 1-888-333-4642 or philip@mercedesshop.com

Then make an informed decision regarding cost.

I only do strict OEM, no conversion, however my Best advice:
Join http://www.mercedesshop.com/shopforum/index.php

Re post your question in
Tech Help  
Technical + Open discussion of Gasoline Mercedes-Benz
http://www.mercedesshop.com/shopforum/forumdisplay.php?f=1

There are several members local to you, who may be interested in helping/having fun, and there are many members who have swapped engines around...