I rebuilt my column a while back and I don't remember any plate being 1/2 to 5/8 thick. The retaining plate shown in the repair manual is around 1/8" thick.
Only the original engine is code correct. Some will use the term date correct, I don't know why that makes a difference personally. I guess it makes some people feel better.
Call Firm Feel, they were extremely helpful when I needed a pitman arm for my 69 383-s fast ratio steering box. Only they knew what I really needed. U.S./Canada 1-360-546-3633