Way back when my Gen 1 was young, I had similar issues with not being able to maintain speed. The problem in mine turned out to be a bad fusable link at the battery. When the engine was under load electrical power was reduced to the ignition. I have replaced the fusable like in that truck 4 times for the same problem. It is hard to think I am the only one with failing fuse links causing undiagnosed power losses.
On an offshot when the engine is idleing wiggle the 3 link wires coming off the battery to see if the engine falters. The wire fatigues inside the insulation.

Kevin, I had not noticed the different OD ratios for the lockup transmission. I would love to find a closer ratio for the top gears in the SR transmission, that needs a fix more than anything I have driven in the hills lately.