[mongocanfly]

Center of crank pulley sits at 20” in the rail and the trans tailhousing also sits at 20”. Is my understanding correct?
[yes] as long as your frame rails run parallel to each other, you want to keep centerline of engine/trans running parallel to center line of rear differential from side to side..being offset is ok

Is having the pinion yoke and transmission yoke offset ok?
[yes]

I have a Ford 9” if that matters
[some furds have a offset pinion anyway]

A buddy at work had mentioned the pinion angle should be down since under torque it moves up
[done mostly in drag car]

from curries website
[ According to Currie, the average car crafter should strive for between 1 and 3 degrees between the tailshaft of the transmission and driveshaft, and 1 to 3 degrees between the driveshaft and pinion. Furthermore, the two angles should be nearly equal (between 1 and 3 degrees), but always opposite]
[With a leaf spring–equipped car, the differential movement isn’t nearly as controlled as that of a four-link suspension, so more angle may be needed to prevent the U-joint from hyperextending beyond zero degrees under load. Sometimes people increase the pinion angle to increase dragstrip bite, but Currie does not recommend this approach for a street car]