Vadim. You covered the subject well. The T425 with a 3:07 FD, 14 inch tires, and a 4,000 lbs car was a good combination. GM had the peak HP for the 427, and 455 reached in the 3,600 to 4,200 RPM range. So higher RPM's in the 4200 range, and above isn't an issue. We have 16 inch tires, and 12,000 lbs as you covered. And a different cam profile. That's another can of worms. Till about 2003 we only had the 3:07, and 3:21. I think there was a 3:42 from Cinnabar? So the Power Drive was a way to get the RPM's up to get out of luging our engines years ago. But as you mentioned. The PD is adding more load on the T425. Then Jim K at Applied GMC came out with some great FD ratio choices up to a 4:10 ratio. These ratios ended the luging. And allow the engine/trans to work more efficiently. Some say the PD,or a lower FD ratio are the same. No. Not at all. Is the PD bad? No. It's better then nothing. In your situation. I'd leave things as they are till you have trans issues. Then change the FD ratio. I'm thinking of going to 4:10 from my 3:70. Towing, or not. I'd recommend 3:70. When I'm pulling my enclosed trailer with a car in it. A 78 Royale 403, and the 3:70 FD. My GVW is about 21,000lbs. As a side note. My mileage went up with the 3:70. As to newer engines at 1500 RPM's at 65 MPH. Their design is another kind of animal. Bob Dunahugh Member GMCMI.
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As for newer vehicles, I've owned 11 pickup trucks since 2000, all Chev and GMC. The two 2005's I had were the last pickups that actually had some getup and go, I still have one of those trucks. Its loaded for bear with a heavy-duty commercial capper and full of tools. I get whip-lash every time I get in that truck.

I traded the other 2005 for a 2010 and all the trucks since have been lazy dogs. Sure they do about 1400rpm or less at 95km/hr. They switch in and out of 4 cylinder mode. When you step on the gas it spends the next second or so deciding what gear its going to change to. I can visualize the little hour-glass spinning in its control computers. Many times its feels like you are slowing rather than accelerating in that time.

When you flip the transmission into tow/haul mode, it becomes a different truck. Accelerating from stop with my utility trailer, it may wind up to over 4,000 rpm in each gear (6 of them) before shifting. Depending on the load it may not get to the top gear and the rpms will be in the 2-3000 range at highway speed. This is with a total load of maybe 7-8000lbs.

According to the trucks display. my 2005 gets about 13.2 litres/100km combined city/hyway. The 2016 and 2017 I have now get about 12.7litre/100km. I don't think the new ones will get as much life as the 2005 with all the shifting they do.