Spyder valve o rings should work, considering the valve is the same.
Your bolt doesn't have o rings does it? As most half block bolts I've ever seen don't, I'll assume yours does not; as such, there is going to be some air getting around the bolt. It's going to be much more noticeable in a half block, with such a short path to vent to the outside of the gun.
Your shootdown may be due to the reg not being able to keep up. When fired slowly, do you still have that problem?