Does my /6 sound right?

cudamark said
If you're running 4.56 gears, that sounds about right but if you have 2.76 or 2.93's, you have one or more problems.

It should be noted for benefit of the original poster that the original rear axle ratio on his car was 2.93 or 3.23, and if a later rear axle has been swapped in it's almost certainly a 2.76 or 3.23.

exhaust restriction or retarded timing (those will also cause the exhaust manifold to crack)

Not necessarily; a car can go for years with a restricted exhaust or retarded timing without the exhaust manifold cracking.