I put 4.10's and a trac loc in my car. I spent around $400 on parts with some connections. I know a guy that installs gears so it didn't cost me anything. But most shops are going to charge you quite a bit to do it. If you have any local message boards, ask if there is anyone on there that does gear set up and you could problaby have it done for $100. If you are going to take it to a shop, you might as well just buy the pumpkin from FRPP as it ends up about the same price. Might as well get all new stuff.
Former owner of 94 Bird with DOHC 4.6L swapped from a mark VIII
2011 CTS-V Wagon
1968 Torino was an original 289, C4, fully documented numbers matching car. Now: DOHC 4.6L/T-56 swapped, Cobra intake, IMRC delete, BBK long tubes and H pipe. Dynotech Aluminum Driveshaft, 9" rear with TrueTrac and 4.11's.