unclemarc-
She wont shorten the life of her engine til she starts USING all the available power. If she drives it like a stock truck, with exceptions being pulling that horse trailer up a 6% grade at 60 instead of 45, she wont have any issues with longevity or dependability. If she drives it like a drag racer off every line in town, longevity may be compromised. But, if the engine is figured to go a million miles, and she shortens the life 30% by hot rodding, she still has 500K left to get there.
Now, I know for a fact that a 91 D350 can be tuned to get 23mpg hwy (60mph w/4. 10 gearing) and still keep up with a chipped 24valver. BTDT. Plus, the old ones are much simpler to work on (YOU seem mechanically inclined) and sound better, IMO.
Now for mods-
Injectors- nothing real hot- comparable to the old PWs or maybe some Bully Dog 50hp. there are a few TDR vendors that sell injectors in varying stages, from 30-90hp, for 1st Gens.
Turbine housing. I really like my 14WG. I'd not go smaller than a 16 w/o a gate if she will tow even occasionally. The 91 I had had a ported 18. 5, and would spool fast enough to light the tires. Plus a 16 is a direct bolt in.
Possibly a turbo impellar upgrade if EGts warrant it. I have no experience with the upgrades from HTT, but they seem to be a good deal. I'd go for the most air for the money. I. E. if they say the 58 and 60mm wheel kits are the same price, or close, go for more air. Better to have it and not need it than to wish for it while babysitting the pyrometer.
4" exhaust will help with EGts when towing. I know, there are some that say it makes no difference until you hit 500hp, but this is based on real world towing and driving, not a dyno run. Heck, even getting that crushed stock pipe out of there will help.
And of course, make sure stuff is up to snuff- injector pop pressures, timing, valve adjustment (I set mine at . 009 and . 019, as per Pastor Bob and a few others' recommendation).
I'm sure others will chime in, but this should get you started.
Daniel