Robert - After looking at your info above and the attached pictures, I can see by the casting number on the bell housing that it is the original 1950 GMC unit. Is the flywheel also the original to the truck, is it a later one that came on the '58 270?
In 1950, the original standard flywheel and clutch would have been a 9" clutch unit (1/2 ton only), with the 10-3/4" heavy duty unit as an option for 1/2 tons and used on all other truck models. Your measured 10-1/4 flywheel clutch face contact (working) surface makes it sound more like you have the non-heavy duty 9" clutch flywheel. If the machined surface exceeded 10-3/4", that would indicate it's the larger heavy duty flywheel.
I think further answers to these questions will help steer the discussion towards the correct clutch disc setup for your existing equipment.
