depends which engine you are intending on using it on. the early 'hemi' CVH's had to have their pistons machined down to lower the comp. However the later lean burn's are fine to bolt straight onto - I used to run mine at 9.6 CR up to 11psi boost no problems (totally stock rebuilt XR2 engine with cheapo headgasket).

Obviously at a later stage you can then swap for RST components (pistons etc).