I have to cast my Vote for HPI as well. I am a converted A/E TC3 shock user. I would build my A/E shock the best I could and then check it against my buds HPI, and I myself NEVER could duplicate the smoothness of the HPI shock, or If I did, theat smoothness seemed to last 1 or 2 racedays, then it would start to fel scratchy, MY HPI shock has been mounted a year, with nothing done to it, shy of an oil change for track conditions, still has the origional seals inside, no leaks, smooth as silk.
Now I have used the TC3 shock in the past on many dirt oval cars, and was totaly hppy with it, UNTILL trying the HPI....I had only one pan car when I bought my first HPI shock, I have 3 now..all of them have HPI center shocks on them.
For shaft length, you can use different A/E shafts in the HPI shock, to achive the proper length if needed,I prefer to use the stock supplied shaft and an RPM long ball stud cap, trimed to length to suit the application.
Useing the long end, also allows for some adjustment of preload, the threads on the shock shaft are twice as long as needed to endure the force pans apply to this joint, just dont thread the rod end on so far, and you will have plenty of room. OR if you prefer switch to an A/E .71 shock shaft...it's a touch bigger diameter, but with about 15 mins, and some 1000, and 1500 grit wet/dry papper, and dremmel, and some mothers you can , "GET ER' DONE!"