Vortec engines are built on the same blocks as any other comparable GM engine of the same era, but use cylinder heads and intake manifold geared toward producing the efficient torque that trucks require. Because of their power potential and reasonable price, these cylinder heads and manifolds have become renowned in the aftermarket industry for retrofit onto non-Vortec engines.
The first thing you're going to want to do is stuff a clean shop rag into each one of your intake ports and water passages to prevent debris from falling in. Remove the valve covers, if you haven't already. Next use some carburetor cleaner, a gasket scraper, a brass wire brush-equipped grinder or an upside-down porcupine to clean the old gasket material from your cylinder heads. It doesn't matter how you do it, just don't gouge the head-mating surfaces. Take this opportunity to clean that sludge out of the intake valley.
If you're doing a stock-for-stock replacement, transfer all of the old fittings from your original intake to the new one. This includes all brackets, sensors, vacuum blocks, pipe plugs and mechanical equipment. You could take this opportunity to clean and install the throttle body, but it isn't strictly necessary. If you're using an aftermarket manifold, use the appropriate pipe plugs to close off any unused ports.
Remove the rags from your ports, and lay the intake manifold gaskets onto your cylinder heads. Vortec intake gaskets usually have integrated O-rings around the water ports; don't compromise the design by using unnecessary silicone. Depending on the specific engine, you may need to use rubber from and rear "China-wall" (engine block) seals, but if not, run a thick line of silicone atop the walls from head to head. Make sure to slightly overlap the intake gaskets on either side. Apply Blue Loctite thread-locker to all of the bolt threads, lower the lower intake into place and start the bolts by hand. Hand-tighten all of the bolts, and tighten them in 30 inch-pound increments in the order your engine requires to its specified bolt torque. Order and bolt torque requirements vary by year. Re-install the valve covers.
Now, simply place the lower-to-upper manifold gasket atop the lower manifold, and drop the upper plenum into place. It only goes on one way. Install all of the original retaining bolts (thread-locker is not to be used with plastic manifolds). There will be three bolts per side on V6 engines (four bolts per side on V8s) and one bolt on the front and rear. Incrementally tighten the bolts to your engine's recommended specs.
Attach all vacuum lines to their proper ports, and harness connectors to the sensors and the throttle cable to the throttle body. Start your engine, check for leaks, and allow to idle for five minutes. Re-torque all bolts to compensate for heat expansion, and go buy yourself a taco.
