700-223: Welcome to the AR world.
The basic components are the same. The 6.5 extra weight comes from the heavier components (barrel, handguard and buttstock). Yes any gas block should fit any light and medium barrel.
If the .308 rifle has many parts you don't like I'd guess it will be cheaper to just buy a stripped receiver set.
If the only part you switch is the barrel and can get a decent price for the original one, it could make sense to buy a .308win rifle.
Replacing a barrel or assembling a complete rifle is not very hard. But you need specific tools, time to learn and work carefully. For a one time job it would be cheaper, safer and faster to ask a gunsmith.
For further gunsmithing questions you may start a thread in the black rifle subforum.
In that case go with a stripped set and build your own. With that faxon barrel and a lighter handguard and stag internals you are set for a nice little 6.5 rig around 9-10lbs maybe even less.
Thanks for the info guys, much appreciated. I suspect you are right. Going with a complete upper and lower, buying the parts I want, and having a gunsmith put it together for me is probably the way to go. Hopefully, we’ll get news on pricing today and I can make a decision over the weekend.
If you have any suggestions for a gunsmith in the Edmonton area who you would recommend, feel free to send me a PM.