You should read the thread parked right next to yours in the forum:
"Help! 66 tire and wheel fail!"
It sounds like your 4.25" backspacing will work, but there are a couple of comments about anything larger than that needing work on the upper control arm shroud for clearance. So the styled steel wheels you're looking at might be a problem.
I'm no expert, just making a comment because I read the above thread before I read yours!!
'65 Convertible, 289 4sp