Capt,n you want to start with a square foundation just like a house etc. I would machine both sides and ends to have flat planes you can use as a reference point for layout etc. At least one end and side. as I stated in a previous post. I have seen kits for engines that had or gave ideas for fixtures to hold different parts for machining, did yours? If not it isn't the end of the world you can get there another way. What kind of mill do you have? Whatever you do make sure the head is trammed in and square to the tabe prior to making any cuts. Do the 12 and 6 and 3 and 9 positions. Then square the vise to the spindle so that all your cuts are square.

Tim