Quick googling looks like a 1000+hp rated CTSV IRS tips in at about $5500. Considering a nicely built 9" with all the suspension work, springs, whatever else necessary will probably ring in at what....$2000? That's $3-4000 i'd be pretty willing to spend to keep IRS.
Nevertheless, your build, your call.
I guess you've settled on an iron block then? Can you even top 4 figures with an al block? I'm not that up to speed on LS capabilities.