Rearend Question

Ok, well I'm in the process of swapping out a Slant Six for a 318 V8.

Now, I'm having trouble finding a 8 3/4 rearend, so my question is this,

Realisiticly, how long will my rearend hold out?(It's a '75 Plymouth Duster, and it's a Slant Six car)

Now, I've got a heavy foot, so I'm going to try and watch that, what else should I "not" be doing? And what can I do to help it last as long as possible?

Many thanks in advance.