swapping in a C4

There are active threads discussin this but basically you'll need to replace your flywheel with a flexplate, probably get the block plate and inspection plate while you are at it from the donor car. Possibly need a new crossmember too, but don't take my word for that. I've got a C4 crossmember that I'll sell you for dirt if you need one, it is from a 67 so I'm not sure if it will work or even be needed.
Don't forget the linkage for the shifter and the kick down.

Nope, your 66 should bolt right up. Some very early small blocks used a different bolt pattern, but from late 65 on up through the 90s they were the same (THANK YOU FORD!).

also, id forget about changing the radiator if its working ok. buy a external cooler for the tranny and mount in low in front of the current radiator. it will work much better and keep your tranny nice and cool. u will need the lines though that run from the tranny to the radiator, u will need the kick down, the shifter (id recommend hurst or other aftermarket) but if you stay stock style you will need the neutral safety swith and wiring for the back up lights and starter. the bellhousing should work just fine. oh yeah, u need a torque converter with a 10.5 bolt pattern and the matching flywheel. also, the converter has to match the tranny. c4's had two types, 24 spline for 60? till like 69 and after that they went to a 26 spline input shaft. You said its a later style c4 so its probably a 26 spline which is good because u have many more cheaper options for torque converters... thats why i bought a tci c4. that pretty much covers it. wait oh yeah, u need the metal vacuum line that runs from the intake to vacuum modulator on the tranny and possibly a tee if you have power brakes. DO NOT just run a rubber vacuum line, it will kink and mess up your tranny, it need to be almost all metal except for each end to connect it. now i think ive covered it all.