My dad had a 1990 Chevy 1500 W/T 4.3L, 5spd. Bought it new. Drove it until ~2004 and ~140Kmi. The only major issue all those years was we went through 3 5spd transmissions and had to replace the 'plastic' tube drive shaft with a custom fab'd steel unit (only on the long bed, reg cab, 2wd, 5spd). We went thru the factory and 2 supposed low mile used transmissions. They start to whine, not hold in gear, then go out all together. From what I've heard it's a bearing on an idler shaft that eventually lets loose on them. For all I know they could have improved this trans beyond 1990. He had the original clutch up to the last trans.
Joel