Canadian GMCs used Chevrolet engines - up to big trucks (in GMC terms, Chevrolet did not sell big trucks). Canadian GMC big trucks used what we (USAers) think of as GMC engines.
As you know (but, other may not): Canadian-car 261s came with hydraulic lifters - all USA 261s came from the factory with mechanical lifters. I assume the 58-63 Canadian GMC truck 261s came with mechanical lifters (at least the 1954 Canadian truck 261 has mechanical lifters).
