Take a leaft form Addo's book, and think not least cost option, but the one that works.
Toyota Hilux SR5 Lift out , which is 114.3 mm pcd Falcon five stud
or BW 78 from Cortina TC to TE 6, which is 108 mm PCD four stud,
or BW 78 from Commodore VK onwards, which is 120 mm five stud.
or shortend EB II - ELII 28 spline BTR 78, shortened 100 mm
The best is the Hilux one in 3.9:1 ratio with LSD and 31 splin axles...pretty solid, and easy to fit, and under every blown up Toyota...
Change of hubs and axles is easy on Cortina. You can drill and fill a 5 stud PCD on a Cortina axle, or swap in Commodore diff.
Holden diff is good, has great disc brakes with internal drum brake set-up whci is easy to fit, and you can drill nad fill to 114.3 mm five stud, or change the front brakes for DBA blanks used for early Mustangs, and drill for Commodore metric 120 mm PCD studs, and adjust the offset to suit your Falcon, and get some really good brakes for very little money.
Any way, don't look for the cheapest diff, use the one that works.
Oh, and by the way, all BW or BTR 78 diffs have an offset, and therefore, a short and long axles.
Very smart people get ute or station wagon axles, such as the tougher EB II onwards Ford 28 spline diff's. These can be can be shortened over 100 mm from there 1527 mmmtrack to the Falcon XP's 1420 mm, by just using two of the short axles.
So a shortened axle is made with the fitting of one axle shaft, for less money. They have really good disk brakes, service back-up, and even the ability ot use an ABS circuit, whic is great if you want to make up a traction controll device at the drags.