i have a 74 duster that had a slant 6. i now have a 318 and a 727 trans.will the stock rear end hold up to a built 318 and trans?  

That depends on what kind of driving you plan to do. If you do anymore than just getting from point A to B, it won't last long. Look at it this way, if you do many burnouts, that 7-1/4 rear end may last just about as long as a set of tires.

Since you're building a small block 318, an easy and cheaper alternative to an 8 3/4 rear, is to get an 8 1/4. They are more plentiful than an 8 3/4, and in your application, may be strong enough to live behind your 318.

The Dart Sport in my Avatar has an 8 1/4 in it, and I am running a 360 and a four speed. It's taking a beating.