I don't know what rear ratio options were offered in Canadian trucks. If your truck has the 3.90 gears it should be fairly screaming at 65. Not that that would hurt it. But if it is not screaming you may have 3.38 gears in it. If that is the case, you don't want an overdrive.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.