3.55 gears for 7 1/4

In all research that I have found over the years, the 7 1/4 was introduced back in 1960 and your parts using 5/16" bolts are for those 1960-61 7 1/4 rear ends. After those production years they began to use 3/8" bolts.
I can only assume that Chrysler found that the 5/16 bolts were not strong enough and made the modifications due to larger torque engines.
My old 66 Belvedere had a 318 with a 7 1/4 and 3/8" bolts on the ring gear, the same as my 1973 Duster.
All I know is that your ring gear cannot be used on later model 7 1/4 rears.