So here's what you're gonna wanna do to make everything work. I'm speaking from experience here because I've just put a carbed 454 in my truck. To run the stock fuel pump, you need to take the oil pressure sender from your original motor and find a good spot to screw it into your 400. The fuel pump wont turn on unless it's getting a signal from that sender that says the motor has oil pressure. That is also the sender for you oil pressure gauge. You also need a fuel pressure regulator with a return line, as the original pump puts out about 20PSI and a carb need 5-7.
Then you should remove your original water temperature sender and move it over to the 400, this way you can keep the factory plug and keep everything looking clean.
Next, You'll need to cut off the grey plug that is plugged into the distributor on your original motor. It should have a pink wire and a white wire running to it. Put flat blade connectors on each. The pink wire will plug into the terminal that says BATT on you HEI distributor, and the white wire will go to the side that says TACH.
All the other wiring should be pretty straight forward. Get rid of the EGR and all that garbage, then cut off the plugs, tape up the wire ends and hide them in your wiring harness.
I'd highly recommend getting a Haynes manual and using the wiring schematics in the back, they were a real life saver. It'll also help if you have a roll of masking tape and a pen handy as you unhook the electrical from your original motor, that way you can label everything and keep track of it as it comes off. It'll make putting it back together and deciding what you don't need to use that much easier.
Hope this helps! I hope I'm not forgetting anything