Wow, lots of naysayers here. It's your car and it's so simple to swap from manual to auto a caveman could do it. Anyone that tells you otherwise probably can't find the sharp end of a screwdriver anyway. Since these cars came with both, the floorpans and firewalls are identical, which make your swap even easier. Here's what you'll need: a C4 trans with flex plate and block plate, an automatic-trans brake pedal, a stock automatic shifter and linkage. An automatic-specific accelerator pedal (they have an arm for the kickdown) and possibly a transmission crossmember (not sure if they're different)
Step 1: remove the manual trans, bellhousing, flywheel, pilot bushing, stock clutch linkage, shifter, accelerator pedal and clutch and brake pedal. The clutch and brake pedal attach to the same firewall bracket, regardless of manual or auto and I can tell you from experience it's much easier if you drop the steering column. Pre-'68 cars need to have the steering box unbolted, but it's still easier than fighting to get the pedals out around the column.
Step 2: install the accel pedal first, it's no picnic to get it in since it's got an extra arm on it now, so not having the brake pedal in or the trans allows you to twist it around as needed.
Step 3: Brake pedal goes in and at least on '68's, there's a difference between manual brake cars and power brake cars. Makes sure the brake pushrod is correct length and the brakes work before going any farther.
Step 4: Bolt on the block plate, then the correct flexplate (28oz on older motors) using Loc-Tite and a torque wrench and NEW bolts.
Step 5: Install the converter. Fill it with oil, place it on the trans input shaft, then rotate it at LEAST 5 times while pressing it in place. More if you feel like it, to seat the converter in the pump. You don't want to experience what happens if you don't. Beg, borrow or rent a trans jack, then install the trans, line up the converter bolts with the holes in the flex plate and thread the nuts on.
Step 6: Bolt the starter on, it's likely the same as the manual trans, but maybe not. Get a chrome one from Summit for about $100
Step 7: Install the shifter BEFORE you put the crossmember in, allow yourself as much room as possible. Blow $5 on new bushings for the shift lever, you'll thank me later. Now's a great time to install the speedo cable as well.
Step 8: Buy repro cooler lines, they fit nice and if your car doesn't have the cooler in the radiator, buy a nice one from any local auto parts source for $50.
Step 9: Bolt up the crossmember.
Step 10: Driveshaft goes in, make your life easier and put a new seal in the tailshaft now or clean up the driveway later.
Step 11: Fill the trans with Type F trans fluid, start the car, let it warm up and recheck the fluid. It'll likely need to be topped off.
Step 12: Test drive the car, look for leaks and if all is good, recheck the fluid level one more time and call it good.
You can find all these parts anywhere, try to find someone that swapped their auto for a manual and get everything you need in one stop. I would've given you my C4 for free just to get rid of it. I'l bet lots more would, too.
