Sigourney was in the frog chorus of The Frogs at Yale, and I think was also in the ensemble of Two Gentlemen of Verona. Lily Pons she's not, but she can carry a tune enough to sing Phyllis' material. More importantly, I think she has the imperiousness and hauteur to carry the role.
Carmello is maybe a smidge too young for Sally, but she's the one I'd most like to hear sing that role. And she's capable of breaking your heart as a performer. After seeing her wildly demented Pennywise in Urinetown, she can also do the madness. Heck, she can do anything.
Groener has the song and dance chops, and there's also something a little sad-sack about him.
Ben's the toughest role to cast. I thought of Naughton because he has a certain gravitas and grandeur, and he'd be interesting to see in Ben's breakdown. Truthfully, I'd love a younger John Cullum to have taken on that role, but I was just thinking about who's around today that could still do it.
As for Gene Nelson, like him or not, "The Right Girl" has apparently never been quite the same without him. He's pretty amazing in the existing footage of that number.
"Gif me the cobra jool!"