from a design standpoint your heading in the right direction. 2 issues i have with it so far. first is not enough tooth contact area for the side gears. i only see 3 side gear teeth engaged by the blocks. i would extend both sides down 1 more tooth to engage 5 side gear teeth. if your not sure what i mean i can edit your picture to show u.
aside from the material the blocks are made of my next suggestion has to do with the actual means of holding them in against the gears. you never did mention the size and grade of the bolts you are using.
my idea would do away with the bolts entirely as they seem to me to be a weak point. i would add material to the inside of the blocks and have a through hole in which the dif pin can slide through to retain the blocks. this i believe would be a much stronger means of attachment.