From what I can recall when I built a few lowers with A5, my two big takeaways were to use the actual buffers that had the patented biasing spring and get the receiver extension with the right holes.

For the internal spring, it appears only Vltor and BCM buffers have it. A5H2 is the “standard” weight as it’s similar to a rifle buffer and roughly equivalent to a carbine H3. If I plan to shoot anything that would have trouble cycling Carbine H3, I keep an A5H0 on hand (equivalent to Carbine H1).

For the extensions, they’re all roughly the same length but the holes for stock positions can be different. If I remember right, the AR10 tubes typically have six hole positions and won’t allow short carbine stocks to lock fully collapsed. Vltor also sells a 7-position stock that addresses this. The BCM stock has, I believe, 8-positions and locks with their carbine stock fully collapsed.