Drive anywhere you want in the barrens or anywhere else for that matter and access the river anywhere you want... this is Maine and the big landowners invite the public to enjoy their lands. You will not see posted signs anywhere for the most part. The law is that if land is not posted with signs annually you can freely enter to fish, hunt trap or whatever.
NOW as to Atlantic Salmon.... Sorry Dude but my buddies and me sued the Feds many years ago to enforce the ESA so you cannot target AS on any downeast River. There are likely no AS left on the Narraguagus.
There are some brokie ponds on Acadia but they are marginal. You might try discovering the Moosehead Region, no access issues there either and it's beautiful country.