Chad,
I believe it' s pretty common practice for hunting in Africa, to stuff an aluminum arrow with a smaller diameter aluminum or carbon arrow. It gets the weight & spine up both. I don' t know what combinations would give you the correct spine, but it seems to me there could be a lot of combinations, including cedars inside aluminum (i.e. 5/16 cedar inside 2216 alum). Anyway, it would take some experimentation, but probably wouldn' t take too long to come up with something workable. As an altrenative, you might look at Gold Tip Big Game shafts, with heavy heads, or extra weight, would be a lot simpler, and more repeatable.

Darrel