There is an old article describing where Ole CS put a 351 Boss with a C6 in his personal Cobra.
I am no longer able to drive a straight transmission and have changed to an automatic.
I had a trust worthy old top loader and really hated to give it up. I took one of the rarer heavy duty C4s down to our local transmission guy who has had several years of racing experience to be rebuilt. We talked for a while and he recommended that I go to an AOD rather than a C4. He felt he could build it as tough or a little bit tougher and that I would appreciate the overdrive. I found an AOD with converter and yolk at the salvage yard for $100 and had him rebuild it. We kept the stall converter at the stock RPMs. The fly wheel had to be changed. The AOD took longer bell housing bolts than the one for the top loader. The trans mount had to be moved rearward about an inch. I bought a TV cable kit, carb mounting kit and shifter. A C4 shifter would have worked fine but I didn't have one. I used an aftermarket trans cooler from PRC. It all went together well and there were no problems with fitment. The drive shaft did not even need to be cut. The speedometer gear fit and had the right number of teeth for a reasonably accurate speedometer reading.
But I did have a lot of bad luck that is thankfully not typcial for others but is very typical for me. Somehow the TV cable did not sit right and burned the clutches and bands in the tranny in a matter of miles. It had to be taken out and those replaced. After it was back together, in less than a few weeks, the pump and rear clutches went out and the tranny had to come out and be rebuilt again. The tranny specialist put an extra block plate in between the block and bell housing to prevent this from happening again. I have only had it back on the road a short time and hold my breath every time the transmission shifts. Ha!! The AOD makes the car much easier to drive and the overdrive is well worth having. The rear ratio is 3.5:1 Not having the clutch pedal in my tight foot box makes braking easier and improves road comfort significantly. Plus I am no longer riding the clutch part of the time.
If the transmission goes out this time and if I don't drive it off a cliff.
I think that I would go to a 4R70W. From what I have read they may be a little stronger and have a little lower first gear ratio about 2.8 compared to 2.4 of an AOD. Plus they can use the after market electronic control sytem mentioned earlier. The down side is that the 4R70W is about an inch longer which would mean shortening an already short drive shaft even shorter. The electronic controler costs about $700 and I believe I would have to convert to an electronic speedometer. And that is after I have purchased and paid to have a 4R rebuilt--at this point not a pleasant perspective.
Oh by the way when I decided to go to the automatic transmission, I also decided to just have the engine rebuilt too and while it was being rebuilt but in a stroker kit. Because I am no longer able to do anything my self, I had to hire others to do the work. If you think the transmission story is bad, the engine rebuild turned into a major train wreck. After it was rebuilt with a stroker kit in it, this 408 cammed 4bl head Cleveland with a professionally tweaked carb, and roller rockers made 200 RWHP and 280 ftpd torque on the dyno. Yeah, it really happened.