When you say you have the factory setup, do you mean factory 3 piece for a 440-6, or factory 3 piece for a 440 4bbl, or what? The factory 4bbl linkage will not just work on a 6bbl setup. the rod on the firewall will need to be lengthened, and a new rod to the top linkage pivot will need to be made. The top rod can be made out of steel round stock and threaded so the old linkage end screws on and allows adjustment. The firewall rod can have a connecting nut and a piece of threaded rod screwed onto it to lengthen it. both of these modifications can be done with simple hand tools and about 5 bucks in hardware store pieces. Let us know what linkage you are starting with, and we can help more.