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If you were doing a 1989 or older 6.2 diesel conversion I could see 26-28mpg highway without special techniques. My old 6.2 with a 700r4 got 26mpg highway without special techniques (but I had 2.8 gears).
The 6.5 is NOT a 6.2, the C code 6.2 was designed for fuel economy period, the 6.5 and J codes were not. Whether you can make a 6.5 into a 6.2 I don't know but the two motors are tuned for totally different things.
For a 6.5 to behave like a 6.2 you would need smaller pre cups, higher compression, a detuned turbo (or a delete of the turbo since high compression + turbo = boom) You would also need to advance the timing.
I would also recommend moving from 16" tires back to the stock 15" size tires the old rigs had. I would also lower the front end a bit (or the whole lot).
To get higher than 26mpg with a suburban you will need to do the following
1. Full Hypermile
2. NV3500 or 4500 MT transmission.
You could try other things but the absolute max for a modded suburban is normally around 26mpg for the untrained with an AT and appropriate gearing.
Beyond that would require special techniques and a MT.
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