Your tires won't support 4750 ft-lb of torque. They'll smoke long before you reach that level.
Even if they could, let's assume you have a 10" lever arm from the yokes to the pinion support....I don't remember how long it is. That would result in a force of 5700 pounds upward at the bolt. Assuming you use a 1/2" bolt there the cross sectional area is .196 square inches. The shear stress would be about 29,000 PSI and a grade 8 bolt can take 170,000 PSI. There's lots of safety margin at the bolt. The urethane bushing isn't going anywhere either. We set the pinion 1/2 degree lower than the engine angle to accomodate some small movement of the pinion. I looked at the stresses on the crossmember as well and long ago went to a .188" thick wall tube. So the design is sound from an engineering point of view.
There are other frame suppliers that use the same type of pinion support...Progressive Automotive, The Street Shop, and I believe even SRG. Newman uses a torque arm which looks nice, but as you mentioned it takes an additional crossmember. If you're using an AME center section the crossmember is already there, but it's placed too far back to use Newman's torque arm. There was a guy here and on the other site that built his "own" C4 conversion (so he says) who actually took my frame jig to his house and used my design...he used the two Newman crossmembers and torque arm. They run around $1000.
So the torque arm isn't necessary, and it does essentially the same thing as the pinion support bracket commonly in use. The reason they used the c-beam on the C4 Corvette was to stiffen the chassis/body assembly. That's also why the C5/6/7 Corvettes use a torque tube.