Hi John.
Welcome to CHR.
I'm restoring a 66 Impala convertible in the shop. I just looked at the frame and I didn't see any extra reinforcement on it. I'm sure that the 65 frame will bolt right up to your 66 body. I don't have a 65 to compare it to so there may be some differences that you could see when you compare the two. I would measure the frame thickness with a micrometer at the side rails. They may have used thicker steel on the convertible frame but I doubt it. The hardtop needed a sturdy frame too.
AL