Unfortunately deadlift gripping strength is almost untrainable, you are limited by anatomy. I never once missed a deadlift due to grip, could probably hold onto at least 100lbs over my max, but several champions have said they left plenty on the platfotm due to grip issues, I think Kirk Karwoski was one of them. The strongmen use straps because no one can hold onto 500 kilos. The thinner deadlift bar was one trick to inflate deadlift numbers but the IPF never allowed them.