The old rear end is probably going to give you a hard time getting out. But dont get discouraged. The brakes lines will have to be bent a tad to fit but the only thing you really need to buy is rear end fluid and friction modifier.
I asume the '85 doesn't have the mounting provisions for the quad shocks, although you could run without them.
Another thing to look into is whether the u-joint is the same size between the two rear-ends.
I question whether you need to modify the brake lines. On my 8.8, hard lines go to the brake cylinders from a "distribution block" bolted to the axle housing. That distribution block is connected to a hard line on the chassis by a flexible rubber line. I would think the 7.5 has a similar setup, and you could just connect the flexible rubber line to the distribution block on the 8.8. The 8.8 probably already has the hard lines on it?
That being said, since Dean85GT obviously owns an '85, he may well know more about this than me.