My rig is a much older rig than what the OP is talking about but it is a good comparison as far as gas to diesel in that I've been driving it for 25 years 20 with the 390/6.4L v8 and 5 with the 3.8L Cummins 4 cyl. The 390 is an ideal truck motor except it's fuel consumption it tows well and works like a mule so no real complaints but when it was time to rebuild it the last time we chose the oil burner route being a '68 ford the 6BT was just too damn long/heavy for my needs plus all I wanted was to do the same thing the 390 did,I didn't need/want a sled puller,drag racer and had no desire to "roll coal" I started with a 105HP VE pumped non inter cooled 4BT the only HP mods done to it were a large I/C and a HE221W 6cm turbo with the pump turned up to match the turbo,the "little"221 is an ideal fit to the 4BT's strong points instant response from 1400 and will pull to the de-fuel point of the 3,200rpm spring without smoking unless you try to make it smoke and even then it's only a very light haze so fairly modest fueling rates. The 4BT easily out works the 390 in every way,the 390 averaged 8-10 mpg the 4bt has been averaging 16-20 doing the same job so for my needs the gas to diesel change was a winner.Now there are a few traits of the 4BT that I could live without it is LOUD and it shakes like it has the DT's and with ~200,000 miles it is a bit slow to start when temps drop below freezing, but overall it's been rock solid reliable and the harder I work it the happier it seems.to sum it up I have 1/2 the displacement doing more work with better reliability using 1/2 the fuel I know the numbers would not be as favorable with newer engines but replacing a late '50's gas V8 with a late '80's diesel 4 banger made sense for us.FWIW