Go to a salvage yard and get a Ford 9" rear end. Ford made them for years and they are everywhere. They are relatively inexpensive, and the shops that do them usually have done a lot of them because they are so popular. They are very hard to break, and very easy to change gear ratios, add spools and positraction to . In my area they charge $80.00 to narrow one, and they can take the stock Ford axles, cut a reweld them for not that much. If you want to go first class you can get the axles from Moser for $399.00. That's really not a lot to pay for a almost bullit proof rearend. Fact is you only need to worry when you start putting five or six hundred horses on the ground. Look for a 78-79 Lincoln Versailles, they are 9" with the disc brakes. The 9"s are also in a lot of trucks.
Also you might want to consider G.M. The older Olds, Pontiac and Cadillacs, amd trucks with the twelve bolt rearends. Dana 44's and 60's are out there too in Suburbans, and bigger older trucks. You might also check out the 81 Cutlass, it most likely has a 10 bolt, but it could have a posi. I've had it happen that I called people and told them the owner wanted the car moved and disposed of, and told them I would do it if they would sign over the title to me. they usually don't want to mess with it, cause it will only cost them more money, especially if there is some storage involved.
