You can get blocks made up, or you can change rocker arms. A different brand of rocker arms will work better on those heads.

I covered this subject in my book. The mfgs make rocker arms in all different lengths. HS makes their rocker arms really long. Put a shorter rocker arm on there and you won't need to mess with shims or welding or blocks.

(crabman is correct, you need shorter rocker arms not shims. You need to sink the rocker shafts into the head rather than raise them.)