Even though my 1967 C10 had no wire connector at all for the radio, you have more than I started with, but I used the OEM type radio anyway. Being retired has disadvantages.
A 1975 AM/FM radio should be push button station memory with chrome plated cast metal that maintains the OEM look and I like it. With all that being said and having more electronics experience than many, I am no expert by any means. I can't even find a place to purchase the OEM type connector for the radio and had no luck in the bone yards. The radio connector is cut off first and long gone. It's like, why is the carb linkage on a used lawnmower gone, bent or replaced with bicycle spokes?
Using your radio, to an amp, requires clean low power output, other wise you get a lot of distortion. Modern GM radios, communicate via class 2 serial data, so if a BOSE amp is installed, the radio automatically drops output for the correct signal to be controlled output from the amp. Matching speakers by resistance is important as well as you can't run 4 ohms rear speakers while have 8 ohm fronts and not expect the radio to fail and crappy sound.
One place to start, is a company I found, that does update the guts in 12 different options, to what you want, while keeping the radio clean original factory looking front. I am not familiar with the 75, but on the 1967 C10, there is only 7" behind the front of the dash. A 1984 Delco from my sons Monte Carlo is almost 8 inches deep.
(Side note; I worked on a 1938 Chrysler for a short time under the dash repairs, The radio was "Tube" operated and the size of a mailbox.)
Try going to and calling Gary Tayman (941) 371-8924 or
www.garytayman.com for suggestions, schematics, connectors, upgrades and also most importantly, what you can expect using what you have now and want to do. Yes, there is money involved but advise & information is cheap. They have a lot of experience with all brands of vintage radios, keeping them looking period correct, but has state-of-the-art guts and add ons that my help you get some tunes. Don't forget your matched speaker upgrades as well... those house speakers take up room for the beer cooler behind the seats.
Best of luck.