instaling speakers on dash

[1968GMC]

I was thinking that while my windshield is out, now is a good time to cut hols on top of my dash (beside window vents). I was thinking of putting 4x6 oval or 4.5 round speakers. Im also taking out my gaz tank out of the cab so il be able to instal speaker boxes with 8-10 inch speakers behind my seat. Has enyone eaver cut and instal speaker on top of dash? If so how do they sound. I know some of you guys and gals dont like the idea of cuting up a nice dash, but that dosent bother me much. Tell me what you think.