How would you set it up for a mil-spec carbine extension, and how would you do an a5 extension.

I am putting on an adjustable gas block if that matters.

mil-spec USGI AR-15 carbine extension, 7" inside depth, or Aero Precision enhanced AR-15 carbine buffer tube, 7" inside depth, at Brownells
Slash's HeavyBuffer 308 AR carbine buffer spring "Custom", $15
Primary Arms Expo 5.3oz 2.5" OAL buffer, $40

If you're looking for complete kits, ArmaLite AR-10 Carbine kit direct from ArmaLite, which are intermediate (A5) length. They're often out of stock. Kaw Valley Precision makes a very nice AR-15 carbine length 308 stock completion kit with a 5.6oz 308 carbine buffer.
For intermediate (A5) length and if you want to buy each component, Vltor has A5 blem receiver extensions on sale. There's also the Magpul SR25/M110 enhanced RE, but have been OOS for a while. Any quality H2, or more common the H3, buffer will work. For springs, ArmaLite EA1095 or the LMT LM308A9. Some like the Tubb spring.

I run A5, Tubbs flatwire, H3 buffer and an adjustable gas block, smooth as butter...

Aero Enhanced A5/RE10 extension to cradle the BCG rear, AR15 sized H3 buffer and an Armalite EA1095 spring.  This setup is on all my AR10s, it's great and replacement springs are cheap.  A couple have an H2 if they have a long gas system (Rifle gas 16", intermediate 14.5") coupled with adjustable gas.

I just wish someone made an AR15 buffer with the preload spring found in actual A5 buffers.