Rock - You truck is most likely to have the 3.90 rear axle as that was the standard regular production equipment coming from the factory. The 4.11 was installed in trucks equipped with the Regular Production Option (RPO) 315 - 3 speed overdrive transmission in the 3100 and 3200 trucks.
The proof is in the stamped characters on the left front face of your 3100/3200 differential pumpkin. The letters "AF" indicate 3.90 and "AG" indicate the 4.11 gear set.
I would second Craig's (TUTS 59) recommendation to use the 3.90 with T5 and the 261. But, that is dependent on what the T5's final drive output is. A T5 OD output of around 0.87 will work best with the 3.90, where a 0.76 T5 will work best with the 4.11 from all the research through other folks setups I've read.
In fact my '51 is slated to get a 261 with the 0.87 T5 and 3.90 rear open drive axle treatment at some point in the future.
