Need to know what the sole purpose of the 3/16ths plate that is needed for this upgrade? Got my backing plates with rotors and calipers (loaded) and all seems lined up? Why would these be needed?
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It should be closer to 1/16" spacers. They are needed because early axles had a 2.81" spacing between the housing flange and the axle flange. The later axles used 2.75". If your rotors are centered in the calipers, then you don't need the spacers. I have noticed that a few aftermarket axle manufactures (including moser and at least one other) sell the 68-72 axles 1/16" shorter than stock which gives you the later 2.75" offset. This may be case for your car.

Personally, I think the people selling the spacers for $40 are ripping people off. You can accomplish the same thing by either cutting your old drum backing plates or simply inserting 1/16" washers in between either the housing flange and the LS1 backing plate or between the LS1 caliper and backing plate. Just make sure you use good washers - your typical hardware store washers are inconsistent in thickness both between different washers and across a single washer.