I did not click the links but I recently installed a new R4 on my 82 300SD and had those flat washer seal problems like you are having.

To make a long story short I had a variety of used but good flat washer seals and through trial and error and an extra can on Juice I got a good seal no leaks.

After all this done, I walked into the local CARQUEST store and they had a nice box of all sort of seals for GM R-4.

This box was $12 and box was see through and like 1" X 1" X 4". A snap box.

I didn't buy it because I did not need it but I'd suggest you look to get this Universal Seal box for GM for R-4.

Try NAPA?

Steve

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1982 300SD