Well, I make the pattern off a tracing on the block. Also you need to trial fit the manifold/heads to see the fit on the back and the thickness. The front can pretty much be flush with the block. I rough them out and then drill the center hole in the block AND spacer at the same time. Then I tap the block and drill the spacer bigger to accept the bolt and attach it with the bolt. Actually I use #8 Machine Screws Allen drive. Then with the center bolt in I drill through everything to tap the 2 outer screws. The location is not critical as long as you do not put them too far to the outside like I did on my Back one. Then I champher the outer edges on a belt sander slowly by trial fitting and eyeballing the edge. It takes a while. I cut mine on a band saw and shaped them with a carbide cutter on a die grinder.