The best diesel conversion I've seen done was in a 1974 C30 dually...the truck's 454 had spun a bearing,and the owner,a trailer salesman that sold campers and mobile homes decided he'd had enough of the gas hogging BBC,and he swapped in an Isuzu straight 6 diesel out of a big "box" truck he had,that hit a low bridge and was totaled..(the foreruner of the "Duramax"??)....the motor fit nicely in the engine compartment,and he also used the 5 speed overdrive the engine came with factory..if not for the different location of the shifter,and the shift lever appearence,you'd never know anthing was changed by looking at the truck,inside or out!..he said it wasn't the big ordeal we thought it would be to convert it..easier than he thought actually...
The guy used to haul mobile homes from the factory in Indiana, to here in the northeast...he claimed he could do a round trip on what he'd spend one way for gas with the 454,and he never had to downshift on a hill anymore--he said it would cruise at 80 mph like nothing while towing!..he estimated his fuel mileage somewhere between 16 and 20 mpg fully loaded!...I'm not sure if it had a turbo,I dont think it did...wish I bought that truck,he offered it to me and my brother for 1500 bucks,because the cab and rockers were getting too rotted to pass inspection,when he decided to retire and sell his bussiness...

...
I've also seen 2 GM trucks with Perkins diesels swapped in--they were straight six motors too..one was for sale at a swap meet for 2500 bucks,and it was being bought just as I walked up to look at it!..the guy had a "spare"motor just like the one in the truck for 150 bucks..I was VERY tempted,but I didn't buy it..