Backing palte to backing plate measurement is not of much importance on a rear axle so long as the springs fit between them.
It is important to have the same wheel mounting surface to wheel mounting surface measurement, or be prepared to find or have made wheels to make it fit you intended application. This measurement is best taken with a tape measure, a helper, and two straight edges.
Educate yourself on the differences between a dana axle and the rest of them.
Don't spend your money till you understand all the info that you can find on the part you intend to buy. Be your own expert. Don't trust any salesman, including a drunk one at the junk yard. Do your homework and buy what you need for the reasons that matter to you, not what the salesman thinks are your needs.
On another note, the price quoted for the axle you are looking at sounds high to me. I am only 3,000 miles away, and that may make a difference, but $300 to $400 would be top dollar to me, $250 sounds very reasonable.
Check eBay for past auctions, ones that actually sold, that is a good indicator of fair price.
Grigg
