I have the M2C chassis on my .3 and it isn't even close to fitting aside from the drop screw plates. I'd have to cut int the receiver box to get the C block to fit.
A new .4 Chassis has been ordered and the only thing that'll carry over from the .3 will be the differentials, and A/B hinge pin holders.

What started in theory out as good plan to only upgrade a couple items on my .3 has turned into an almost full on .4 conversion, things weren't as compatible as I thought they would be