What "collector" said makes sense~If the ground isn't flat~it can be made flat, in one spot you're not going to keep moving it around```That's if you build a frame```
If you install two 6"X 6"s~ the 2- upright 2"X 6"s could be held on top of the 6"Xs by upright 2"X 6" upright braces~ Bolted~one each side of each 6"X 6" (2' down on the upright post & extending up 6" ) so as to make a saddle for the two 2"X 6"s to sit in~ the 2X6s will be through bolted in place (using galvy All thread ) in addition: the the 2X6s can be bolted a couple places in line```
The posts can be pressure treated and still painted~nothing above ground needs be```
Any part of the bench swing you think needs replacing, bolted and in where you think it should be exterior glued``` when finishing use a grinder to touch the surfaces old and new~rounding corners and grinding away split edges```I'd use Linseed oil, if it be me, thinned slightly so it will go in deep~if and when you paint it I'd use a good Alkyed stain so it doesn't come off like news paper after several months~If oil stain is thinned it won't go on too thick and dry like skin and peel```
You can make this thing look and feel like a monument~it won't feel like it will fall apart~you'll need a good concrete footing around the posts not just a couple inches like a fence post~a good square block```
We'd like to be there for the making and swinging~but```