It all depends on what you have now. I'm not familiar with what came behind your straight six, but I would replace it with either a t350, or something with an overdrive if you can afford it. Or you can go with a stick shift, and in that you would need to get the pedals and linkages. I think the SBC will bolt up to the 6's mounts, but if not, SBC mounts for your truck should be easily accessable. As for the rear in the truck, it will work, but I recommend either getting it rebuilt (if it hasn't already), or going to something beefier like a 12 bolt.
Also, how's the condition of the truck now? Is it currently running? The amount of work needed depends on how long it has sat. If more than a couple of years, check the gas tank and lines. The tank needs to be flushed and cleaned. The radiator will need the same, or even a replacement if it can't keep the 327 cool. There's other little things, but that's most of the majors. Good luck!