Homer, In addition to my own 400gr cast bullets I too used Von Gruff' 350gr cast bullets that he sent me. Still using some of his when I shoot the 404. I loaned my Hoch mould to VG when he was getting started with his 404 so he could have some cast on hand when his rifle was finished and ready to shoot.
I followed suit with the AR2209 load he developed for his 350gr cast bullets. He got about 1900fps out of his rifle while I got 1800fps with the same load. My Mauser 404 has quite a bit of freebore so I would expect my velocity to be less than VG's Mauser with his custom chamber. Your velocity seems quite low from that load

I too found that with the components I used that I got very slight delayed ignition, only just perceptible but there nevertheless. Solved that by using a wool wad seated on the powder. These wads are/were available for purchase from The Cast Bullet Kid, see link below. I also use his Beaver grease bullet lube. I could get clover leaf groups at 50m from this loading when I was on my game.

http://www.beavergrease.co.nz/TCBK.html

350 gr Hoch Cast GC
AR2209 powder
63.0 grains load
CCI 250 primer
RWS case
COL - 3.490"
Wool wad