I have a friend who uses late 60s impala b ench seats in his builds due to cast. You are correct they work well with minor tweeks.I have 70 Impala 2 dr hdtp bench seats, both front & rear in my 57 2 dr hdtp & they fit fine. I had to fab. 2'' riser blocks for front, but was a fairly easy job. You can still buy new seat covers for these also.
An early Impala seat about that vintage was in mine when I got it. Fortunately I located an original '55 seat and replaced it.I used a split bench from a 63 Impala. Close to same size as trifive seat. I used trifive tracks made a few simple mods and bolted it down to normal location.