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My '77 K20 has 4.10:1 rear gears and 31.6" tires (265/70R17). It also has a THM350. I have run that on highways for countless miles. Yes, it is a little high-rev'ing due to the gearing, but I can run it 70 miles per hour all day. 70 MPH is about 3,100 RPM for me. For you with 33" tires, that same 70 MPH should be just a hair under 3,000 depending on converter slip. So...you should be able to run 65 no problem. 65 should be somewhere between 2700 and 2800 RPM.
Marine 350's will cruise at that RPM all day! As long as you don't have a cooling issue, it will be fine.
Erik, how many years between them? I had a 68 Chevelle with 4.88/th350 that I drove from Phx, AZ all the way to Bowling Green KY, and it didn't bother me a bit....nowadays, I'd be stopping at every motel along the way....
☝️ That's what I was worried about, I was almost fully on the shoulder and the Tacoma had maybe 100 yards before a head on collision before he whipped into my lane! One of my biggest issues right now is a driveline vibration. I can't run faster than 60 or it feels like the truck will shake apart. I already had the rear drive line completely gone through. And while it helped my pinion angle is off because of 2.5" lift springs I just installed. I'm going to abstain from highway driving until I get the issues fixed so I can run 70 smoothly. Maybe that way I won't end up with someone up my rear bumper like happened with you.