Um....ok...what was the topic here
About the 6.5. I'll include some general experience in addition to comments on the engine since the 6.5 does generally come with a GMC truck.
My Dad has a 1993 non ECU turbo diesel 3500 (SRW) 4x4 and it seems to be a decent truck. The Short commings are;
Generally Harder to work on, (access to injection system is MUCH harder and you will have to pull the turbo and intake manifold before you can replace lines, injectors, or injector pump depending on what side of the engine you are working on)
Cooling package is under designed. I nearly overheated the thing once because I was used to driving my F150 (302 needs to feel the floor before responding when loaded). Luckilly I backed off before hitting the red. New head gaskets and quad core rad and it seems to be doing well now (6 years later).
MPGs are not great at all. All time best was 18 MPG, and average seems to be 15-16 (believed to be 4.10 gears with stock 245/75/16 tires)
TURN RADIUS!!!!!!!! for the same wheelbase, the chevys of this era suck for manouvering compared to fords. Make sure you take this into account because this is one thing my dad hates about his truck.
Rust around heat shields and exhaust manifolds. Can't be helped, it will happen eventually
ABS sucks,
Front wheel bearings are non adjustable throw away units,
CV shafts instead of U joints on front axle drive (4x4 only), but in fairness, I never replaced either of them.
4wd thermal expansion actuator is expensive, delays 4x4 engagement, and failure prone (google it)
More external plumbing for the oil cooler and heater core so the engine is more prone to fluid leaks
Advantages are:
Less prone to fuel system air leaks,
smoother idle (if that matters at all)
4L80E transmission. stock for stock compared to the E4OD, I consider the chevy to be slightly more reliable. Although once you start modding the engine all bets are off. Don't know how the manual transmissions compare but I believe chevy did use a DMF like ford.
The 4L80 also doesn't have an overdrive cancel switch, but instead has a full shift quadrant with OD, D, 2, 1. Its really a matter of preference though.
The 4L80 also comes with 3/8" TRANNY COOLER LINES. HELL YEAH!!!!
Access to the thermostat is VERY easy,
Access to the CDR is also VERY easy,
Acess to the turbocharger is, yup you guessed it, its not hard to pull one compared to ford's setup,
good glow plug system with solid state controller that is not as expensive to replace as the ford equivalent,
Electric fuel pump, some consider this advantageous,
Serpentine belt can be replaced in seconds with one (1) tool, you don't even have to get it around the fan.
Hydroboost comes standard on these years (I think)
AC is very powerful and GM has been making better ACs that ford for a long time (I am sorry to admit). Recently had to replace the compressor cycling switch and it still works like new.