I've done only a couple of those cars, but I've done many Bonnevilles, LeSabres, Sevilles, Devilles.....they're all on the same platform; electrically, they're the same car. Except the Olds has the ignition switch mounted differently in the dash.
--The constant power at the ignition switch will work fine, as long as you're not installing extra-draw things like multiple sirens, window rollup modules, air horns, etcetera.
Since the ignition-switch constant wire is fused at 10 amps (under the hood, I think), it would be wise to remove the 30-amp fuses from the harness for the relay pack, and replace them with 7.5 or 5 amp fuses........that way, if your aftermarket wiring shorts out in the future, only those fuses blow........leaving the underhood fuse intact, and you can still drive the car.
--The brown radio-accessory wire doesn't need to be powered........ you have enough ignition/accessory/starter outputs from the remote start relay pack to power up everything else.....no extra relays will be needed.
(With the brown wire unpowered, the wipers will be inoperative when the remote starter is on, but that's a good thing! You wouldn't want the wipers straining against a dry windshield, or worse, stuck in ice, if you weren't there to turn them off.)
--I'm no fan of the 457G, but it'll do the job. It doesn't do two-stage unlocking, it doesn't turn on the domelight, it doesn't turn on the exterior lights at night...........on your car there's some extra wiring you have to do to prevent the "security" light from coming on............
the Trilogix GM-DLBP is a FAR superior piece, and it'll do all of that stuff, with no flaws, and no fancy wiring. The GM-DLBP also gives you a door trigger output that catches all four doors.
--For door trigger, hopefully the 457G has an output for it? that way, you won't need diodes, or relays, or fishing wire into the doors, or any of that.
You will be tempted to use the underdash courtesy light as a door trigger. That would work, however, it only operates at night.
--The negative-trigger parking light wire can be found coming right down the steering column; no need to chase down the DIM.
Make a note NOW to move the Clifford's parking light jumper to the negative position.....I am not sure if you'd cause any damage to the car if you mistakenly sent positive power to the car's parking light wire.
--I'm pretty sure the brake pedal switch will be there under the dash staring at you; no need to remove the kick panel if you don't want to.
--The ignition wires are REALLY tight to get to in this car.......plan for this, and allow patience. You can reach behind the ignition switch, unplug the plug, and that'll allow you to pull the wires straight and down, to make it just a little easier.
I tried removing the radio and climate control panel to get to the ignition wires from there, but it didn't help.
You might be tempted to skip installing the starter-kill feature, as that requires cutting the Olds' starter wire in half, and I wouldn't blame you. The PK3 should give you reasonable anti-theft protection anyway.
--You may or may not have noticed, but your car has a feature where it'll start perfectly, no matter how short or how long you hold the key to to the Start position....go ahead and try just bumping it, or holding it there a long time....it'll always start just right.
For that reason, you'll be okay if you don't want to use the underhood tach wire....it'll always start reliably in voltage-sense mode.