You can try getting a MOA rail fitted 5 moa will raise your shot by 30 inches 10 will raise it 60 inch 20 moa 120 inch
Minute of angle = MOA
Shiming the front will increase the drop moving the point of zero closer so shim the rear mount instead with a piece of photo film to raise the poi

I don't think the tube holding the cross hairs will go up that high, and shimming the front base, I don't know. What you need is a new cross hair with the dots below the cross, They will raise your line of sight past the 350 zero. Or,,,,, just put the cross at the top of your target, 600 yards without the right scope is a long shot. I hope you have a heavy barrel target rifle in 308 or bigger. My M42 is unbelievable, but, It will shoot better than me all day long and I rarely go over 200 yards. I tried the 600 yard line once, didn't like it. Oh I could hit the target, but I like hitting doorknobs at 200 yards better. What are you shooting anyway? I hope your not trying to stretch out a 223 to the max.