You'd need to move the stock box out a long way to the side of the chassis rail if you want it to have clearance for headers. I had this problem on my truck, so fitted the Saginaw setup from a later truck, the fitting kit came from CPP, it was very simple to install.
There's some pictures on the Chevy Truck page here:
http://www.millpower-race-engineering.piczo.com/ good luck with your project.
Stuart.
