you need a flexplate froma 351w with 164 teeth, 28oz balance factor, and an 11.4" converter bolt circle. you can get one from summit for like $80. as stated make sure the TV cable is adjusted properly, and you need an aod crossmember.
1) The AOD is slighltly (approx. 1.3") longer and quite a bit wider than the C4. For the most part, it will fit in the same space, but it may require "massaging" the tunnel on early cars (pre '67).
2) Cost? This depends... A used high mileage AOD can be had for cheap (under $200), or you can pop for a rebuilt ($600-$1200) or built unit for big cash (Lentechs start around $2500). Converters cost extra, but a decent performance converter can be had for under $400.
The actual conversion can be cheap or pricey, it just depends on how you put it all together. Assuming you source everything from a place like Windsor-Fox(rather than fab it), you can plan on spending around $500. Assume $120 for a crossmember, $100 for a TV cable, $80 for a flexplate, $60 for the shifter adapter, and add the correct blockplate, inspection cover, and yoke. Obviously, that doesn't include things like new speedo gear, or odds and ends like fluid, filter, gaskets, new mount, etc. Your old starter may or may not work... if not, add $50. Obviously, if you can fab the crossmember, source the TV cable from Lokar, and get the flexplate from NPD for $50, you'll save some money.
Oh, and you'll probably need to have your driveshaft shortened... I'd guess probably another $100. I didn't have to shorten mine, but my car is a '70 and I went from an FMX to the AOD.
3) What will it take? Simple: remove the starter, the tranny, the converter, flexplate, blockplate, kickdown, etc. Install new blockplate, flexplate, tranny, converter, new crossmember, yoke, starter, inspection plate. Measure for the correct driveshaft length, get the driveshaft shortened, install driveshaft. Install TV cable, adjust TV cable (using correct pressure guage on correct port). Install shifter adapter and adjust. Install speedo. Test and choose new correct speedo gear to correct for speedo error.
Obviously, this is the cliff notes version of the swap... assume it is more involved and more work than it appears here. Also, if you do the swap, DO NOT DRIVE THE CAR BEFORE ADJUSTING THE TV CABLE CORRECTLY, OR YOU WILL BURN UP THE AOD. AODs are very sensitive to fluid pressure, and will literally burn up in less than 5 minutes if its not adjusted correctly.
Hey my c4 seems to be goin to the pits and i'm lookin for another c4 or aod to swap into, i have a very mildly built 302, it's got all the edelbrock performer parts from cam and lifters to double roller timing chain set to performer manifold and performer 1406 600cfm carb and some 150 buck shorty hedmn headers, i'm not lookin for anything massive on the power capability scale, but with handling range at least 300hp as i will prob end up with alum heads one of these days. I don't know how to adjust my kickdown with my new carb setup on there, if turn the screw and put the kickdown farther from the throttle linkage will it shift earlier or later, i just got my fresh motor to run last night and drove it around my block a few times tryin to adjust and figure it out because i'm getting like massive slipping in second, like it won't engage unless i give it at least half throttle, and if i lock the tranny in first it engages a little late but feels like it may be slipping, i didn't ride it hard, just a few trips around the block to try and fig it out, some help please!