Well, it depends on the definition of peice it together. If you buy a fast or electromotive, accel, holley controller or the like, you would still need to get the right dizzy, and sensors for it, but you can get just about any intake, throttle body and injectors for it (the right inpedence). Holley does sell complete kits, so does accel. Not sure about fast and electromotive and the like. Megesquirt isn't bad but it is really a DIY kit. Th upside is you would be using GM/FORD sensors and what not. So if something takes a crap, you could go to autozone and buy replacements.
If you have almost know knowledge of how EFI works, and what everything does, i would get a complete bolt-on kit. They usually have base fuel/spark maps, and you can go online and download bins, the files the ecm uses, that are close to your engine.