I have a 6/284 and have used H-4350 in it quite a bit. Mine is a 26" 10 twist shaw. I screwed up too. I should have built an 8 twist..
I can tell you mine will not shoot anything bigger than a 85gr Sierra. I have tried 90's 95's and 105's and all were very unacceptable. Mine likes the 70 gr match kings the best for small varmint. But will shoot the 85 game king well enough to be very effective on speedgoats.
So if I were you, I would start with a 75 V-max , a WLR primer and about 46 or 48 gr of H-4350 and work up slow. I will warn you too the H-4350 will get plenty of velocity and can be hard on brass. So start low.