I did practically the same thing up Afton Mountain in my 170 CI 63 Valient. Had to downshift to second gear, then back to third until speed dropped to 35 MPH, second to 50 then third again, rinse and repeat.
Got 28 MPG in that old Valient. Probably had the carb set a little lean.
Oh yeah, bias ply recaps
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regards
Mech