[LongBox]

It seems that your truck is all together, so cleaning the firewall will be tough, with the distributor, brake cylinder, steering column, heater box, etc. all in the way. And, not sure, but I think you want to save the factory chalk marks, etc. Challenging to say the least.

I pressure washed the inner fenders and firewall on my truck, and it made a substantial improvement. I made sure to keep as much water as possible away from anything electrical. IF you do this, then maybe Simple Green or a degreaser intended for food processing plants and restaurants would remove more discolouration. Ultimately, I think using a polishing compound is going to be required.

Keep us informed on your progress.