ANYTHING older than 95 and most 95s are going to be OBD-1 or in GM speak ALDL. The sensors and overall engine configurations are going to be different, and not compatible. Even between years sometimes little things change that prevents some parts from working on some engines. When I look up a compatibility chart of the 4.3L engine, the long block shows the same from 96-02, but different in 03. Not sure what's different in the 03+ long block, but it's a different part number, and cost.
Which ever engine you do get a hold of, make sure to keep your original intake, and accessories. Those did change from year to year, as the engineers fixed flaws, or developed a new trap for that mouse.