I was using .043 soft copper on that engine, but i no longer O'ring the blk or put reciever grooves in the heads.
I now use the ICS copper with the embedded stainless O'rings, They were using them on twin turbo engines, pushing big boost so i figured they would hold a blower motor as well. I haven't had any trouble yet and they are around $200.00 a set, much cheaper than O'ringing and recieaver grooves. i know the top fuel and top alcohol cars are still o'ringing but they are pushing 60 lbs boost with the screw type blowers, my blower is 1471 roots, built by mert Littlefield (RIP) i had known Mert since the mid 80's, when i had a 540" blown Rodek engine.
Zip.