Any reason for the 300blk and 6.5 desire?
If you have separate complete uppers, pretty much every AR using a milspec lower will mate with any milspec upper.
The 300blk uses all the same parts as a 5.56 AR, but for the barrel... If you have a complete upper set up in 300 ready to go, you just need to put it on and go. Even though the mags are interchangeable... It is recommended that you mark or color code the magazines you will be using, as obvious 300 only, as 300 will chamber and fire in a 5.56 rifle, resulting in a violent disassembly, and possible severe injuries. Dedicated mags help prevent this.
The 6.5 needs a special bolt, proper barrel and special magazines. With a complete upper ready to go, you just drop it on and grab the correct magazine.
You really need to narrow down exactly what you want from the rifle, do you want free float (with 6.5 that seems likely), do you want a trigger that is better than mil spec? Plus other questions that need answered...
After that, its easier to point you in the right direction.