The problem I'm having is when I get close to 70mph it will kick down then back up on shift make the engine sound like its screaming for mercy and speedometer will drop down to about 65mph but feel like I'm gaining speed
It depends. My '94 C2500 HD Cab and Chassis had a speed limiter programmed into the PROM in the ECM from the factory. 82 MPH was it. No, I didn't run out of revs, 3.42 14 Bolt full-floater. Didn't matter if I was in 3rd or OD, converter locked up or not. I could have 5,000 lbs in the back, downhill, with a 40mph tailwind and it would defuel to maintain 82mph.
Which wasn't all that bad. When traveling I-80 all I had to do was mash the pedal to the metal and let it rip at a constant 82, up hill, down hill or on the level. Nice thing was the Speed Limit was 75mph and the State Patrol generally runs a 7mph "cushion". Since it was a Cheyenne base model C&C, no cruise control (or power windows or locks, just a vinyl bench, rubber floor mat, AM/FM) so I could maintain a constant speed just wedging my foot against the hump on the accelerator pedal.