Gauge issues are common on certain manufactures gauges/dash panels. A common fix is removing the panel and re-soldering the joints on the back of the panel. They are prone to cold solder joints.
Your decision between the two drive trains is tough. I love the 5.9. I have owned several but in a full size bus I held out for the 8.3. The 5.9 with the 3060 backing it up is not a bad choice. There are a handful here on the board and they seem to be happy with them.
My 8.3 will last longer, have more power and is cheaper to rebuild than a 5.9
Like the man said: "There is no replacement for displacement".
Hmmmm..... I wonder if I could stuff an 8.3 into my pickup????