No, they don't. The six cylinder assembly is identical and interchangeable with the four cylinder version except for the length of the slave cylinder pushrod.
The pushrod slides out of the rubber and the old one slides right into the rubber and against the piston inside. My 2.5L Kammback has the entire system swapped from a 1983 4.2L except that pushrod and the tube resevoir.
I have personally purchased a new six cylinder master cylinder from Rock Auto multiple times. I have gotten a wrong one. I've gotten a large number of right ones. For some time a few years back I could not get any at all from Rockauto or any other place because they stopped production. Since that time period they've become available again and I got several. It is unreliable, but not a showstopper. I've now got six extras for that reason.
You wouldnt want to sell me one of those extra would you? This way i could trust that im receiving the right one..