Any year 2WD C4 or C5 will work. A C5 is a C4 with better oiling and a lock up converter. I like C5s better, but the valvebody is different and there is not that much in the way of shift kits for them. 70+ valvebodies from a C4 interchange, but you have to use a C4 convertor and bellhousing though. If you plan on much more than 400hp a 70+ will be a better choice since they have a larger input shaft.
The 2 C5s I have taken apart had beefier clutch packs in them than the C4s I have seen. There are 4 different size bell housings for a SBF. 164 tooth flexplate fits the large C4 and all C5s. 157 tooth for the middle C4 and 144? for the Mustang II. The MII bellhousing is rare and will possibly not work with a Mustang. We tried to use one on my brother inlaws 65 Falcon and the starter would not go on. The bellhousing, flexplate and torque convertor must all match. The C5 has a deeper bellhousing and thicker convertor because of the clutch in the convertor. You will also need one with the correct shifter arm on it if you plan on using a stock shifter.
Mixing bellhousing, flex plates and convertors of different sizes is one thing that can cause a snag, and the wrong spline in the convertor is another common tripping point.