I also have a 3600 and understand your challenge with finding information for the 3600 rather than the 3100. My project journal is kind of long but may provide some 3600 specific insights.
I currently have the stock suspension but have upgraded the tie rod ends to more modern variety and replaced the drag link with an adjustable one but that was in large part due to my engine swap requiring relocation of the steering gear.
I also looked for a 4.10 center section for my 4.56 rear axle withotu success so I converted to a 14-bolt rear axle using the process outlined in a Tech Tip on this website. That provided newer type rear brakes but still drums.
I know there are a lot of T5 transmission projects but I chose to use an NV3500 to get a 5 speed with overdrive. I had a custom drive shaft made to connect the NV3500 to the 14-bolt axle. I chose to install a SBC engine but with my transmission, axle, and tire combination I am running about 2100 rpm at 60 MPH.
You may want to look at yout seat springs as I assume that you still have the original bench seat. I converted to '90's vintage Chevy truck seats to get integrated seat belts but also improve the seat performance (in my opinion). I was always planning to relate the fuel tank but the seat change made it mandatory (no space left for the in cab tank).
I installed an under bed fuel tank but because of the narrow frame rail spacing of the 3600 was limited to a 16 gallon Tanks Inc. version. I chose to run the filler out the passenger side through the bedside (below the floor) with the filler located in the back side of the rear fender.
I do not have a lot of miles on my truck as it just got to the point of being road worthy recently. I do drive on country roads that are anything but smooth and flat and don't think the ride is that bad and my steering is pretty good even bouncing around at 60+ mph.
If you have any questions about things 3600 specific please feel free to send me a PM.
