I may be mistaken, but i seem to recall that the Canadian Pontiacs from the early '50s used a 261 with a 235 head as factory equipment.
That swap will increase the compression ratio, due partly to the smaller chamber, but also from the larger piston displacement. As far as the valvetrain goes, I don't think that the guide hight will make a difference, you'ld be using 235 valves. As long as the sprins were compatible with the cam, there shouldn't be a problem.
