I've never owned or driven a 455 in anything but I have years of experience with 400s and several different cams and from what I gather talking to other people it sounds like that cam is a bit on the small side for your cubic inches. I currently have the Crower 916 in my 400 and it is a mild cam even for this motor. Just guessing but I'd bet it doesn't run too much better than your 400 did. 400s can be made to run real strong but it takes a bit of work. 455s with a few of the right components, from what I understand, run real strong in an Fbody right out of the box.
Personally I think you rear end ratio is killing any chance you have at a decent street car. I had my 3.23s go bad on me and swapped in some 2.97s for about a year and with my 4sp it was all I could do to pull away from a stoplight with out double clutching. It really was not as enjoyable to drive around town.