66 c10 diesel engine options?

[Haaland]

Hello everyone. I'm looking into the possibility of a diesel swap. What I'm looking for is something that can get 25-30mpg+ and up to 80mph on the highway. I swapped to a t5 and would like to keep it, with that said no more than 300ft of torque. Emissions is a problem either. Some of the engines I have been looking into are OM617, TDI, kubotas, and some farm equipment diesels but I'm afraid they will feel to underpowered. I know there will be some fabrication for adapter plates but that's part of the fun. Does anyone have any suggestions on other engine options? I know there is always the old Chevy Diesel's and 4bts but I want to do something a little different. Thank you in advance.