I am in the process of doing just that in a 78. I have a long way to go yet, because I am doing alot of frame work right now. (Not related to Big Block install, just beefing up the entire frame)
The Big Block will bolt right in, but you will need all of the BB accessories brackets for the alternator, A/C etc.
The Big Block weighs 125 lbs more than the small block so the front will sit a little lower. You can opt to install Big Block springs for an early C3 Big Block.
You will need to get a larger radiator for better cooling.
Also, the driverside valve cover will be right on the brake booster, so you may need to custom fit the valve cover on to the engine. One option people have done to remedy this is to replace the brake booster with a HydraBoost. This is a hydraulic way of boosting your brakes.
The hood clearance may also be tight, so you might want to look into getting something like an L88 hood, which looks real sweet anyway, and helps with the cooling.
If you are using an auto trans, you might think about beefing it up to handle the torque, or upgrade to something like Turbo 400.
Let us hear what you decide to do!
Bill