I had this problem last year at Lodi. I started using tang sights but crossfires twice because I could not read the number board so I put the DZ scope on the rifle but even with a 30"barrel I saw the barrel shadow looking through the front globe sight. I took the front sight off but still see the shadow.
When I ordered the A scope I forgot to order a high block set to raise the scope enough so I can put the front sight back on for using the tang sight at closer ranges.
I went to town this morning and bought a 12" caliper

so I hope this solves the problem with mounting the blocks.
Could have bought a lot of lead for what I paid for this measuring stick