With how few miles you are going to put on this vehicle (just using it to get supplies and such) I doubt you will ever even come close to breaking even with an engine swap. I know that you mentioned lowering your emissions and all, but maybe you could take that money and invest it in some other area of your life to lower your consumption. For most folks, switching over to LED light bulbs in their homes is going to negate carbon emissions many, many times over what a diesel swap is going to save.
Don't get me wrong, I love these sorts of projects. Diesel powered vehicles have a lot of advantages over their gas counterparts. Sometimes those advantages will never outweigh the cost/effort/time/headache that it requires.
Maybe giving us some more data, such as yearly mileage expected, load (weight) expected in the box, trailer you will be pulling, and how long you are planning on keeping the vehicle would give us a better idea on what to suggest. For example, I have an early 90's Dodge 1 ton van with a gas v8. It gets terrible, terrible mileage. I have considered dropping in a Cummins/manual transmission several times. But when I consider that I only drive the van 2-3,000 miles a year to buy/store milk replacer, it doesn't seem the best use of my resources.
Good luck on whatever you decide. I will follow this thread to see what you can eek out of this old girl, but having realistic expectations going in will give YOU a better idea than what WE can give you flying blind.
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