I would just use pure Lead...and a good Lube of course.240+ weight for a 45 Auto in a style that feeds and functions well is a handloaders dream. Working up the perfect combination of components would be a labor of love. For my own use I would cast a softer alloy than #2 for use in a 45 Auto or a 45 A.R., that would give me a slightly heavier bullet of the same dimensions and make it even more desirable.
As an aside: You can call RCBS and see what they say. They offer superior customer service.
What are these Bullets diameter wise? Have you miked them?
240-ish Gran would be a very nice weight for .45 Auto.