Any changes that require new parts would decrease your "greenness".
As stated above, most modernizations are not necessarily "greener".
However, a simple conversion to natural gas (if that is possible) might be considered to be "greener".
If you are leaking oil, replace gaskets/seals; but, first consider the old saying about these trucks, "if they are not leaking a little, they are out of fluid". If the engine is burning oil, then consider rebuilding the block/head. The consensus seems to be that hardened valve-seats serve no purpose on 216/235/261 engines - high-compression engines might benefit from hardened valve seats.
Look at it this way, you are not recycling, you are not re-using, you are using it longer (which is very green in the case of this vehicle).
