Questions regarding rear lower cab mount bushings for a 91 Chevy K1500 Pickup

Hi everyone,
I purchased some new lower rear cab mount bushings or cushions for my 91 Chevy K1500 Pickup with a standard two door cab. The rubber
bushings have what looks like a thin flat plastic ring bonded on one end. On the other end, there is no plastic ring, but the outer edge of the bushing has a small fillet or radius.
What I need t know is, which end is up? Do I install the plastic ring against the frame bracket, or does it go against the round steel retainer / washer that is directly under the head of the cab mount bolts ?
Also, I would like to find some bushings that are tougher than the stock GM bushings. Does anyone know where I can purchase just two lower polyurethane bushings or is it always necessary to buy a whole bushing kit for $50.00 or $60 bucks ?
I would appreciate any feedback.
Thanks John
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