I had a 79 E350 with the 351 and C6. It got 9-10 mpg. I also had a 91 E350 with a 351 EFI and AOD. It got 11-12. They were both extended models, but not high tops. A diesel swap, even if you got 20 mpg, would take quite a while to recoup your costs. Diesel usually costs 50-75 cents a gallon more, also.