Stick with the small valves, the bore on the 326 is quite small. Be aware that you'll have to port match the 140 heads to match your intake. This involves removal of about 1/8" of overlap and blending down into the port to work properly. Depending on your desires, a cam with around 210/220 duration @ .050" should work well with the rest of your parts.

The 140 heads are the best choice for a daily pump gas driver as long as you clean them up to work with the intake. Proper tuning of the carb and distributor is highly recommended.