One thing I do to make the longer chassis more stable in the rear is to move the rear hubs forward a little bit, not all the way forward just 1-2mm from centered. It puts more weight over the rear end and plants it better.

Also, I move the upper front arm all the way forward and yes it increases off power steering, but it makes it less prone to spinning out under hard accelerations out of corners. Diff oil sounds fine, everyone thinks diffs are the way to tune, they aren't, the geometry is better, then alter the diff oil from 5-7-3. I like 6-8-4 in the summer and 5-7-3 in the winter. I mix for the front and center, not perfect but who cares, but I did buy some 4 for the rear.

the rear shocks on most Mugens I see are way too stiff on the oil in the rear. I run slightly drilled white pistons with 35wt oil. I suppose if you don't want to drill them you can use 30 or the one in the middle of 30 and 35. what is it 32.? ...and take a little droop out of the rear to make it level with the front. and lower the rear ride height below the front.

and if the battery is up front put it in the rear.