The 68 a bodies had Dana 60 in them to make them legal for super stock with Dana the automatics had 8 3/4 also to make that rear end legal for super stock. Had to be produced by the factory not just something you dreamed up, after enough were produced then you could build one out of any /6 or whatever.
The original cars used 66 B body width axels which area if I remember 1 1/2 wider 3/4 per side, when you see photos of back in the day they have a funny looking rear wheels. There are some errors in that specs sheet, 12.5:1 compression was always standard in race Hemi, there was no modified horsepower rating, NHRA did this after car started stomping the hell out of everybody, sandbagging only last so long. Chrysler always put the (who me) face on and rated horsepower low when introducing a package car so did other manufacturers. The clutch should be a 11" but may have been a scalloped to save weight.
I believe the front brakes were 10.5 drums also to save weight.