The GM TBI-to-Vortec cylinder heads intake manifold is hatefully expensive.The only mods you need for vortec heads are an intake manifold , the adapter for the Tbi unit, and an exhaust manifold if you plan on keeping EGR.
Plus the EGR pipe/hose, and the fittings on each end.
You'd need to fabricate a bracket for the throttle, cruise, and Trans TV cable if used.
Or the Summit-brand (Made by Trick Flow) TBI heads which are a genuine bolt-on except for needing hardened pushrods.The only other performance option is the Trick Flow 56cc heads, which also requires a new intake and an adapter.
[Edit] The Summit heads have a 62cc chamber, so compression drops a bit on a 305. Supposedly "angle plug" heads, but no-one is complaining that the headers or exhaust manifolds don't fit. And one guy mentions that you'd need a couple of pipe plugs for a coolant port machined into each head.
The heads come with guideplates, if you use the guideplates you'd need hardened pushrods and different NON-GUIDED rocker arms. If you use the OEM guided rockers, you don't need the guideplates or the hardened pushrods. [/EDIT]
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