I don't know for sure what rad came with the 455. I do know that many use the 4 core with 400's with some success... I use a brass/copper 4 core on my modified 400/468(same as previously unmodified 400) and it has been adequate during the cooler part of the year(since November). It will get a more thorough test as the temp rises over the next month or so.

That 455 is going to produce alot more HP than your 350...and consequently more heat. Others may disagree but, I don't see how the rad designed for the 350 will cool the 455.

Just my opinion...