To give others an idea of what it takes to install a 455 in a G-body I am posting the progress of the swap. It's easier than you think stuffing one of these under the hood.
I started with a cleaned out engine bay and a 7a headed '72 Olds 350 that was built in Q1 of '73 (notice the rare factory bronze 'Oldsmobile' valve covers, these only came blue in '73).
I didn't bother to detail the engine bay since it was a daily driver and the mosquito fogging 307 needed to be quickly swapped so I could get to work.
The A/C, cruise control emissions and ECM were ditched and a heater box was installed. Notice the red wires wrapped up on the right side, they are both power wires, one is for the ECM and the other is for the A/C. These can be saved to run anything requiring 12v.
Don't be deterred if you want to save your A/C system, a heater box is not required and there is plenty of space under the hood still for the 455.
Here is a link on how to save your A/C system:
(yes I know the PCV valve is disconnected...)
I completely removed all necessary wiring by following wiring leading from the ECM up to the steering column, cruise control, fuse box and gauge pack. Instead of having to fabricate a plate for the ECM harness hole I just sawed the harness flush with the rubber grommet on both sides.
I also found a non-ECM kick panel from an 84 Diesel Cutlass Calais with the same Sand Gray color interior to clean things up further in the interior. Will post some pictures later.