You might not expect to find a built C4 automatic under this shifter.....
However, having a bum leg has forced me to have to have to convert a number of my cars (including my HiPo Mustang, BOSS 351, and ERA 289 FIA) to automatic trannys. I've had very good results with a C4 (I used a small-block C6 in the BOSS 351, and had an AOD built for my ERA that I never got around to installing).
I shift my C4 with a 289 Cobra shifter grafted to a standard C4 shifter....
The rusty looking part is where they are welded together (with the boot installed, it doesn't show). The original lockout mechanism is functional. I had the two shifters welded together with an angle to the left, so that when the shifter was in "Park" position, it would look like a 4-speed parked in "Reverse". I have no shift indicator, I shift it by feel only.
I believe your best bet is a built C4. The C6 small-block is big and heavy, and uses up a lot more horsepower to operate. The AOD or AODE is big (an inch longer than a C4 and a good bit bulkier), heavy, and may require the addition of electronic controls.
After experimenting with a number of different setups, I finally have my C4 shifting exactly like I wanted it to, which is the firmest, hardest, tire-screeching shift into each gear. Along the way, I tried a full-manual reverse-pattern valve-body, a fully automatic valve body, a semi-automatic valve body, and eventually the one I have now which is a forward-pattern full manual valve body. My preference leaned toward full manual due to the complications of plumbing vacuum lines to the Webers for the automatic shifts.
I bought my stall converter and full-manual control valve body from Broader Performance
Broader Performance in Texas. Your desire is a bit different from mine, as you want to be able to shift automatically in Drive, and override manually when you want to. For that type of functionality, I have had very good luck with using shift kits from Trans-Go
TransGo-Home Page. Their kit will allow full automatic operation with manual override whenever you want it. With that kit, you do not get automatic downshifts, as you eliminate the downshift control cable, which also prevents "surprise" downshifts in a corner.
I have run these C4s in open track and autocross events with good results. Maybe not quite as exciting as rowing the gears of Top-Loader, but, they will get the job done.
Jeff