Totally!

I agree. As I see it, Anthony was miscast in a lot of rolesheh.

Funny thing is that I had the exact same thought at the time. Kudos.
I can eat 50 eggs - Cool Hand Luke

I agree. I think some of what Franciosa did in his acting was good but he didn't look like anybody from the South or from the Varner family for that matter! When he said, am I your son, you could almost think that maybe Varner's wife had a little fling when Varner was out of town and Tony was the result! And maybe that's why Varner didn't seem to value him as flesh and blood.

couldn't agree with you more bing another scene I loved him in was with Woodward when she thanks him for protecting her when she was a little girl
"why are you married to him then if you can't work with him how do you live with him?"

Kind of agree except for in one scene, when he confronts his father about if he ever loved him, I thought him and Welles were both magnificent in the scene.

I totally agree about Franciosa. I saw this movie for the first time in the same place you did at about the same age. It probably was the first time I saw Newman, Welles, or Woodward act. (But I recognized J. Pat O'Malley from "Spin and Marty".)
Franciosa stood out in every scene as just not belonging there. But let's give him a break - even well-cast, there was no way he could keep up with the skills of the rest of the cast.

"Franciosa mostly succeeded."
Not so much. I completely agree with the OP.
I can eat 50 eggs - Cool Hand Luke

"New York street hustler" is an unfortunate term, yet it certainly shadows some of Franciosa's roles. Difficult to take the Street out of the kid. Yet in this role, I'd have to disagree. Yes, Franciosa was from N.Y. and trained there, but his accent sufficed and he was in character as the misdirected son of a powerful man. For a 111 minute movie, not a lot of time to develop his supporting role, and the late 50's direction he acted on, Franciosa mostly succeeded.