Howdy back:
'65 to '68 and most '69s will have the 52 cc small chamber heads. Castings Casting with a C5, C6, C7, C8 and C9s- other that the "M" suffix casting. These heads will have 1.69" intakes and 1.38" exhasut valves.
You can fit 1.75" intakes and 1.50" exhaust valves, but they will be of little value because of shrouding in the smaller chambers. The larger valves would be a bigger help at higher rpms. If I were you, I'd opt to stick with the stock size valves and have hardened seats installed. The biggest gain in valves takes place within an inch of the seat. That's why I suggested a three angle seat and back-cutting the intakes. That along with a little port and bowl clean up and polishing the chambers will get you a long way to better flow.
If you do opt for the larger valves, spend some time unshrouding the chambers to take full advantage. The plus of the smaller chamber heads is a higher quench to bore ratio, for better turbulence and combustion efficiency.
Again, your tri-carb set up will take care of many of the single carb issues on the top end.
Adios, David