Deer rifle...
JSF--If you've had great success with the 7.62x39 then mebbe youshould stick with that. It will put down deer just fine under most conditions. Something else of similar power in an AR-15 would do the same.
OTOH, if the range is extreme, or some other condition is far less than ideal, the greater power available with the .308 cartridge might help make up the deficit. Keep in mind that no cartridge on earth will make up for poor shooting--whatever you use you'll need to practice, practice, practice with. But mebbe I'm preaching to the choir here.
Anyhow, if you want the greater oomph provided by the .308, then by all means go with the AR-10. If you're building this rifle specifically for deer then perhaps that's the way to go. Was it me, that's what I'd do.
Whatever you decide, please keep us posted. WITH PIX, please. We all love pix!