Plastic socket might be workable but I can't see the '72-up socket working correctly in the '70-'71 side markers: too deep and probably won't seal correctly. Go to the back of the car and take a look at the separate socket used in the rear side markers (access from inside trunk). Those weren't used up front because of sealing issues. The crimped sockets didn't entirely prevent water ingress; most of these are presently in one stage or another of corrosion, often pretty bad. In your situation I would probably cut away the crimped portion and remove the socket, then look at what size hole I am dealing with, then pick out an appropriate snap-in socket (plastic or metal) and seal it in place with a bead of RTV silicone. Wedge-base vs. bayonet-base bulb doesn't matter. Consequences of corrosion are less with wedge-base bulb (bulb doesn't light) than with bayonet-base bulb (bulb doesn't light and doesn't come out of socket) -- that's if you use original brass-base bayonet bulbs. If you use nickelplate-base bulbs, no such problem.
Difficulty is, it looks like there are very few candidate sockets that
would maybe work better than the '72+ variety with the '70-'71 lamp housings. Try a NAPA Echlin LS-6520 (nice big flange for a good RTV silicone seal). Takes a wedge-base bulb. Use 168 or W5W (2825) in preference to 194. If you want super bright, use 2886X or 3652. Make the front ones flash with the "cross feed" method; see
here.
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