George,
California could be a tricky place to get a cheap rear end. You may be able to find one at one of the pick and pull places. The speacialized places will want a million bucks for one.
I know the 55-9 rear is a good swap, and those 3.90s will be great behind a T-5. But be honest with yourself. If your you sure you don't want a posi rear, continue. If you do want posi, you might look at other options. If you want a few more options for gears, a 55-59 gmc would have a spicer 45 axle. There might be more gear choices available for that axle. But you would have to change the whole axle.
I am doing a rear end swap in my 1940 gmc, and decided I wanted posi and disc brakes and wanted to keep the six lug pattern. The sollutiion for me was to get a F*rd Nine inch floating rear end, 12 inch disc brakes, and 4.11 torque lock posi. It may be overkill, but I know it will handle everything I throw at it.
