98 Z-71 GM tech help speedo correction

Hoping you can help me. I have a 98 Chevy K1500 ext cab 5.7 engine. Just had new tires put on. Went up to 295-75-16 from stock of 265-75-16. By the way, they fit just fine with no lift or raising of the front end...Just left my local chev dealer disappointed and not believing what I was told. Took the truck there after they assured me they could recalibrate the computer for speedo correction. Technician told me at the cashiers counter that he could not calibrate to a 295-75 and the closest he could match to it was a 31-1050. When I look at the dia. at BFG website I see the 295 @ 33.5" have now
265 @ 31.8" stock
31-1050 @ 30.7" calibrated to
Also I asked if the OD(4th gear) could be adjusted so it did not come in so early. This I was told could not be done on this model year. On a lite acceleration it goes into OD at 35mph. My 96 K1500 was perfect by my standards and went into OD at 45mph (it was adjusted by a dealer years ago).. Can you shed some insight into this so I can have the correct adjustments made.. If anything I would like the speedo to read higher than actual since my 18 year old drives the vehicle a lot and It might keep him from paying out $50 to the locals.