That is a neat hunk of iron. I've never messed with interior pieces on those models, but on the later d-line ('69-'75), the dashpad is secured to the dash by a number of phillips head screws. Very simple to remove. I could be wrong, but I just can't imagine that slightly earlier model as being much different. As for a replacement, that could be tough. You might look into repairing yours. I've seen videos on youtube where folks have done some pretty passable repair jobs to their difficult to replace dash pads.
Yes, accessing the speedo cable where it attaches to the speedo head can be cumbersome. You really can't see what you're doing most of the time. Sometimes you can make like a contortionist and come at it from below with your back on the floor. Or, with the seat slid forward and your butt parked sideways on the front edge, you can sometimes reach up under the dash or perhaps through one of the vacant gauge holes. It can be done. You might have to think outside the box to find a method that works for you.
Finding a seat cover to fit your bench seat shouldn't be that difficult. The dimensions are going to be similar to any other fullsize pickup of similar vintage. Anything that would fit a late 60's to early 80's GM-Ford-dodge full bench should fit yours well enough.
