Buying a 727, what to look for?

You want to find a 69 and up transmission. 68 and prior can be identified by a single post nss. Your car will be wired for the 69 and up 3 post nss. You cannot just change the switch as the valve body is also different. 64 and prjor have a flange on the output that the driveshaft bolts to (as do later shorty motor home and truck transmissions. You will want the long tail shaft slip yoke style.