It seems like I do my best thinking in my sleep!  Woke up this morning thinking about rear axle gear ratios.  After researching the Alfa BB it seems that the '79 spider came with a 4.56:1 gear ratio.  In 1980 it was changed to 4.10:1.  I think I want to stay with 4.56 to preserve acceleration for autox.  I have also been following a build on the Locost board using Volvo drivetrain.  The builder of the Volvo donor claims a 240/740 volvo rear axle is a perfect fit.  Now I just have to find out what ratios were available on those cars.