CMS, one of the negatives with working with castings is if you screw one up, it is hard to recover. It may cost you another casting.
The first thing I would recommend for you, especially on the main casting, is to do a layout of all the details (holes & bosses, etc.) on the casting. As I recall, some of the locations of the bosses are not quit on print location. If you do a layout it will allow you to move the entire pattern to more effectively fit the casting.
Another hint I have used on all of my cast engines is this. Once I have established a decent layout the first thing I machine is a flat bottom on the casting. This can require a somewhat difficult setup. Once it is setup and the surface has been flattened, I drill and ream a 1/8 shallow dowel hole on each of two sides of the casting. It can be either side to side or front to back of the base. Then I drill the 4 corner mounting holes on location.
Next I take a 1/2 or 3/4 thick piece of steel or aluminum that is a little larger than the base of the casting, square it up nicely, I drill and ream the two dowel holes and drill and tap the 4 corner holes on the same location as the casting.
From this point on, you can fasten the casting to the plate and do all your maching setups with ease. Once you have a difficult setup, you can remove the casting from the plate and replace it on location without disturbing the setup. I think you will find this very helpful and will be surprised how many times you have to repeat setups.
Looking forward to following this build.