I'm sorely tempted by that 10" DSG 300BLK  upper here:

https://dsgarms.com/dsgu-300-10-mcsr10-agb-a2

However, I'm not sure about the length of that rail.  Will I be able to mount my AAC 762SDN6 on that thing after swapping the FH?  Is there enough clearance to the handguards for the can's QD latch?

I can't promise you for sure, BUT I have the same seekins rail on my 556, and Have a 762sdn6.  My muzzle is not recessed, but I took a picture of it for you anyways.





There is a slight gap around the can and the rail, enough so that I think the can can mount INSIDE the rail, but it would require a thin tool (fork, knive, screw driver tip) to press on the latch when you want to take it off, but possibly doable.