After you have the focal length, do the necessary measuring of the tube and locate the focuser and spider and put it in. Then put the primary and holder in at the approximate location that the numbers say, not drilling it yet, and go find out where things are for focus. Use a 25mm EP and set the focus so that the focuser is about 1/4" from the top of the travel. This should give you enough in travel so you can then use a 4mm EP without running out of focus travel.
After you have gotten the primary in the right place, you can then drill the holes to fix it in that position.
FWIW, I prefer to affix 3 blocks in the tube so that I screw the mirror mount to them with the screws available to the back of the tube. This allows me to remove the mirror without losing the collimation. I also don't put the 3 blocks are exactly 120 deg. apart so the mirror mount is indexed in the tube.