If your 289 has the common boltpattern it should be good to go. The early ones had a 5 bolt boltpattern and require a matching bellhousing.
'79 Mustang had the dual sump pan with a dipstick in it. When I priced it the thing was outragous from any local parts store, places on the internet have them cheaper. You will also need the pickup for the oil pump and actual dipstick to go with it.
A couple of those "while you are there" things is the oil pump (about $20) and driveshaft (less than $10) Pretty cheap insurance IMO, because even if you catch a problem before it causes harm... you gotta pull the engine to fix it. Especially the driveshaft.