I read it was because they wanted a stiffer cam, better able to deal with higher pushrod loads (at higher HP and/or exhaust brake). The small journals limited the max diameter that'd pass thru the bearing...
I was amazed to read someone here is critical of adding a turbo to a naturally aspirated diesel. There are a lot of positives with few negs. I'd be more critical of a factory that sold an engine without one. Adding one is easy and doesn't cost much.
There are other ways to cool pistons though its unlikely it is even necessary on a low HP config. Turbos blow relatively cool air thru the chamber. Without additional fueling you have no additional heat. This additional cool air is basically inert (though it expands when heated so contributes to power production even though it doesn't burn (think- nitrogen side of a N2O set up). Extra air lowers EGT too. The more air the better. Cleans up the emissions and increase fuel economy. Boost on the intake stroke pushes pistons down too. That force creates a small amount of torque. That's free HP. Add some fuel and you get more. If you don't go wild you can get more hp combined with lower thermal stress than the naturally aspirated start point.
I'm not critical. If someone spends time & money to put one I respect that. I just don't understand what you want to achieve. A cool whistle noise when your going to 7/11 to get a slushy?
Turbos don't blow cool air, unless the air has been substantively cooled through an intercooler. Sucking in and compressing air heats it up quite a bit. Thus you want a intercooler to thus "cool it down" the more cool dense air one packs into a cylinder the more power you'll get during the powerstroke. An example would be the cooler on the old Big Cam engines that are part of the intake manifold. The median in which air is cooled is engine coolant, so if that engine is warmed up lets say the coolest air is around 195* that being pressured into the cylinders....pretty warm. That not as efficient as today's intercoolers using the ram air effect.
I don't believe compressed air is enert. Exhaust gases are enert. They don't reburn. But compressed air with fresh oxygen will.
The heat of combustion is pretty intense. The aluminium pistons aren't cooled by the coolant flowing through the heads or block, the liners are cooled that way but the pistons are cooled by oil (thus, oil jets are needed). I'm sure the crank flings a bit up that way but surely not enough.
My belief in this theory is I had a customer come in with a 1973 Hayes COE tractor that was converted into a motorhome. It was armed with a NHC250. He put on jakes, had a Big Cam 2 oil cooler installed, he had the same exhaust manifold like in the 900 series trucks but what he did was made a custom flange to accept a turbo off a 290 (I think). It was put on right at the end beside cyl 5/6 inline. He did this because he ran a cab-over and couldn't put a traditional pulsed exhaust manifold on like on regular front nose trucks would accommodate. Also note he had no pyro/boost gauges for some reason.
Complaint was a heavy knock.
Took the last head off and discovered cyl 6 liner was scored and the wrist pin blue (from heat) and the piston wrist pin journals worn out. All he did was boot around the mountains in BC here on vacation a couple weeks a year. Not sure of what pump work he had done but as a recreational vehicle I wonder why he ran this set up. the valve guide seals where toast too, oil cooler was full of metal, the cam was scored but he didn't want it replaced so we let it go.
New liner, new piston, seals/gaskets/bearings, oil change, set the lashes, labor blah blah blah....$12500 bill. Have a nice day. No warranty, as it's modified horribly wrong. I did put a pyro on though.
I suppose if you run a turbo on a 250' and lightly use it will perform. If I do that and use it for lowbedding I'm quite sure it won't.