Cam selection help please

A couple things here - That 268 is an excellent cam for a street 383. I run them too and they are hard to beat.
Here's a couple stats for you -
You have a static compression that is uber-low. Like if you're lucky it's 7.5:1 depending on the heads and gasket choices. That's based on an 85cc chamber (open chamber that was milled a little, or as-cast closed chamber) and any of the flat top replacement pistons that dont come with reliefs.
The heads you have are more-than-likely not cut for dual springs. older model cams did not require the higher spring loads a fast rate does. So the previous owner could run a number of much larger cams with signle-w-dampner springs. In order to run true duals, you will need to have the guides cut and the spring seats cut on your heads. If you look at the springs now with a flash light you should be able to tell if the seats and guides were cut. If not, prepare to pull the heads.

I dont think a bigger cam will get you more power than that xe268. I run them with 9.8:1 in 383s and they make past 400hp. You may want to look into heads with smaller chambers to get you more compression rather than cam. Cam without compression is more detrimental than helpful when compared to compression without cam. I really would advise against this until you have a better understanding of what you have, and can manage the weak points first.