Upgrade questions

I am looking at buying a unique rig as an RV. I have a couple of questions: Engine is a 80ish 855 BC Cummins 400. Not sure which series. My understanding is that is doesn't have that many miles on it. Probably less then 200k. What sort of upgrades can you do to an old 855 without destroying the reliability and fuel milage?
This vehicle has SLHD axles and a RTO-013. Gearing is 4.11. I am looking for it to cruise at 70 turning 1800 or so, maybe less if possible. Tires are 24.5x11. The whole thing weighs between 30-40k. It isn't a truck exactly but has truck running gear. It needs a Jake too I think. Any ideas?
Kevin

The old 400 Cummins is good they are so so on fuel the 13spd transmission works good behind them.

They are pretty much good for 400hp I wouldn't want to-go much more with one.

You do know the old Cummins are really blue belchers so if you want to use this for a RV make sure you get used to the smell of diesel fumes

As for a JAKE I think you still can get JAKE heads for them the Jake works good on 400 Cummins when they are adjusted properly they also Bark pretty loud.

If I was looking for a RV project or a truck in general if it was Cummins powered the N-14 would be the answer. I do like the 3406 Cat good engine the old Mechanical and Full electronic version are good.

What is the brand of truck ?

If its a old Louisville Ford the 400 Cummins is shoe horned under the hood and its a ****** to work on.

Not that I've done it, but this was copied from someone that did:
drill out the ball in the throttle shaft and back out the screw a bit

adjust your aneroid valve so it gets rid of the fuel supply delay to make it more responsive ( the aneroid's function is to limit fuel until the turbo spools up. Its behind the little tin cover beside the throttle shaft).

shim the governor spring

change the fuel pump button (I think its actually called an 'idle plunger' or something). This is the biggest bang for your buck. Its not that hard to do but I can't recall the specifics because its been over a decade since I did this. A few hand tools are all thats required to pop the cap off the end of the pump then a socket and needle nose pliers to pull the stuff out. The 'buttons' are stamped with a number on the groove where it is narrower and the lower the number, the more fuel pressure. The numbers go up in a weird sequence if I can recall --> alternating between the increase of 2, then 3, then repeat --> like 5, 7, 10, 12, 15, etc. Depending on how far you want to go, guys used to take whatever they had and drop the number by 10 or 15 and it was pretty safe. Just be sure to have a pyro (that works properly), be sure to warm it up properly because that extra fuel will score a liner much quicker when not warmed. Do not lug the engine - drop a gear soon as there is enough rpm room to do so. Someone had an '84 Big Cam III and it had something like a 30ish button and we ran a 5. It was 'hot'!
The fuel pressure increased from somewhere around 180 to over 300. Some guys had a hard time with turbos when they did this.