Bam! This is why I'm always raving about Bob and the EBL... awesome service, even with unrelated questions to the customer. Sounds like you're using the diagram from a Camaro?
I went through the wiring for a '87 4.3l Monte Carlo. In that car this is connector C434.
D & G This is ignition switched power. Make sure that it is ignition, not accessory power. Uses a 7.5A fuse and powers the injectors.
J ORG I couldn't find this terminal. Orange is usually battery power.
R This is from the 3A cranking fuse in the fuse block. It is hot whenever the starter is activated. Lets the ECM know this.
I wouldn't go by that Camaro diagram of C207 either. As terminal B for the '87 Monte is power back to the fuel pump and as an input to the ECM.
N is shown for the P/N switch input to the ECM.
Stock TBI will definitely be very short on fueling... it will have to be tuned for that 90hp difference (considering the stock tune is for a 200hp engine), because you will need more injector/pressure to feed it, and with a stock tune you're stuck at either too much fuel at idle, or not enough at WOT.
The EBL comes with a nice little utility that helps with calculating fueling, BPC (injector base pulse constant), and a few other things, and it looks like you can run 454 injectors (TBI injectors all interchange with each other) at stock fuel pressure to get the fuel you'll need... but it may run rich at idle, which is why it would need the tune.
For exhaust you may be able to run LT1 exhaust manifolds, I'm not sure about interference with the crossmember though. They're good enough for a 300hp production engine, just a thought