Don't stiffen up the front bar. If you do you will make it understeer even more. Use a 500E bar in the back and leave the front bar stock. I think the 400E already has a little stiffer bar in the front than the other 124's due to the added weight of the V8.

Many people think that if you get rid of body sway, you are making the car handle better. That is not necessarily true. What you want to achieve is handling as close to neutral as possible. On any car stiffening the front bar adds understeer. Stiffening the rear bar removes understeer. If the body sways, that's no big deal as long as you are getting out as much understeer as possible. Most all production sedans of any manufacturer have built in understeer for liability reasons. Unskilled drivers won't get into trouble driving an understeering car. They will feel the front end plow and back off. The 124 MB is no different, so your goal is to get out as much understeer as you can.

Believe me on this, I have been through all sorts of bars on my 124 and the stock front bar with a 500E rear bar, takes out about as much understeer as you can get out without having a car that jars your teeth out.

See if you can get your money back on the front bar. If you put it on, you can expect the car to plow more than it already does.

Enjoy,