Head and Block Machined??

[TONY!]

Yes, both need to be machined to get an RA of lower than 50 for Cometic (as their tech staff recommends) and even a lower RA for some other HG makers.
Lower RA's = smoother surfaces. It is not like with sand paper ratings: with RA's (roughness average), the lower numbers are a smoother surface.

Some do lap the block, but I am very much against that because lapping may not create quite as flat a surface as milling. Not only must the surfaces be smooth, but they have to be very flat.

Another method:
IF you really wanted to use a MHG without having it machined, some people are using Permatex Brake Quiet and spraying that on to help aid in the sealing since this material will conform to very small surface irregularities.

Some do just go ahead and not go by the manufacturers' recommendations of having the block milled. Some of those have had luck--although it is not recommended that way. Although some had luck without milling, it is not the book as the proper way to set up for a MHG. You would be taking a chance not going by the proper recommendations.