I don't have a better suggestion for the housing-to-body mounting (considerably different on my '63). Your thought sounds like a reasonable approach to me.
Metro Moulded Parts lists the tail light lens gaskets for the '66, in case this is something useful for you:
https://metrommp.com/tail-light-gask...talina-mp-956/
Not cheap by any stretch, and I have no idea as to how well these fit, but Metro is a reputable company.
GL HTH