If the rated power is 3,500 it's perfectly safe to go 10% above it for shifting - they didn't treat them like Ming vases when they were work trucks. Momentarily buzzing the engine won't break anything.
When to shift? When it runs out of power.
6.00 × 16 is about 28" OD, with 4.11 you're turning 3,500 RPM at 71 MPH.
S = .002975 × RPM × OD ÷ axle ratio
