I have never had to retorque following the initial torquing procedure so your issue may come down to one or more of the following.
1) some of the bolts may be too long, test for them without the washers, if they thread all the way by hand, only being stopped by the head of the bolt against the aluminium head then they should be fine.
2) the threads in the barrel are toast, I’ve known of 2 which have failed in the area you mention. Both started off with 5/16” rears, then I took them out to 3/8”. They then were helicoiled. (Might be worth noting that all 5 of the bolt Holes in my current barrel have now been helicoiled. I never had an issue since doing this.
3) either the head or barrel surface isn’t flat. Use as thick a gasket as you like, if it’s not flat then it won’t seal, copper gaskets aren’t a wonder stuff that fills the cracks.
4) the bolts being used are just shocking, the threads are a bad fit and are of poor quality.
5) or... you need to re-address your head torquing procedure. Take them all up about 5ftlb at a time, don’t go beyond the torque limit. Then slacken each one, one bolt/nut at a time and retorque before moving onto the next one. If you put a mark on the head and the head of the fitting you will see it actually
Goes further. (This is not the same as torquing another few ftlb) the setup will have settled, and your just re-applying the correct torque to the settled part. This is on par with doing a 500mile retorque.
Now let’s all have a beer
68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)