I am also thinking of a 1/5 scale using a mbx-5T as a platform
 

I have been thinking about this for some time. Basically stretching a stock mugen 5T 4-5". That part is easy enough w/a bigger chassis, longer center driveshafts. My issue is longer arms to bring it better to scale. Something like 1-1.5" wider arms for a total 2-3" wider should bring it closer to scale.

Now in saying that, what mans. makes longer arms that would fit w/the 5T? I don't have the exact measurements right now. Anyone have any thoughts? I could have some made out of AL

Well you might be able to use some 6T arms? Or perhaps RC8T or 8ight-T 2.0 arms? Not sure if they'll fit, maybe some modifications... dunno.

I've got a RC8T that I've thought about going to 5th scale with, needs about 7" to get to the same proportions as a Baja 5T, about .25/.5" narrower though. But alas my questions concerning custom parts hasn't gotten answered.