I've had a TKO 5 speed in my '72 'Cuda since I put it together a little over a dozen years ago and have close to 60,000 miles on it so I am pretty familiar with both it's faults and it's virtues. Mine has .68 OD and a 3.27 1st. gear with 26 in. dia. tires and a 3.55 sure grip 8 3/4 rear. First the faults. First gear has more gear reduction than you need or want, it's a pretty good jump to 2nd. This was one of Keisler's earlier "E" body attempts and the first transmission they sent was unusable due to very poor fit. Unlike some other peoples experience Keisler made if right and sent me an updated version without argument and at their expense. The newer version still required a bit more tunnel surgery than I would have preferred in order to get proper driveline angles, but it wasn't something that someone with experience couldn't handle either. The earlier shifters weren't very good, but the later ones are decent. Keep in mind that "E" bodies require an external bolt on shifter so this dosen't help. As a side note '05-'14 Mustangs also use a semi-divorced external shifter and you won't find anyone who owns one saying that the stock factory shifter is anything better than OK. I own a '14 Mustang 5.0 6 speed manual GT myself so I know this from experience. I would be reluctant to try a full on, no lift power shift with either of them. That said with a little practice you can shift either of them pretty quickly. Now the good.I can cruise on the interstate with the 'Cuda at 70-75 mph at 2,200-2,300 rpm which is about 1,000 rpm less than I would be turning with the factory 4 speed and only a touch more than my Mustang. The TKO is slightly noisier at idle than a production transmission, but it's not objectionable. As for clutch action, I am not sure what you call stiff. I am an old guy and I grew up with some pretty stiff Borg & Beck style clutches in my old '63 Sport Fury and my '66 street hemi and I,ve never been mistaken for a body builder. My '14 Mustang has a hydraulic throw out bearing and I think it's really light, but you can go on Mustang forums hear people complain about a stiff clutch. I run a 10.75 Sachs diaphragm clutch on the 10.5 bolt pattern in my 'Cuda and I think it's pretty light and easy. What I am going to say next is somewhat controversial. Contrary to what most people think you do NOT have to remove the large overcenter spring from the clutch pedal in order to use a diaphragm clutch so long as you install a positive and preferably adjustable clutch stop which you can do for a couple of bucks worth of hardware store parts. In fact if you do remove the overcenter spring you should install some sort of return spring in order to keep the throw out bearing from riding constantly on the clutch fingers. As for autocross, you can see from my pic that I have a little experience here. Don't be concerned about shifting, most courses will only require a single shift from 1st. to 2nd. Only once did I ever require 3rd.