OK, so yesterday I was able to deck the bottoms of the "feet" of the base castings first. It was a tad awkward in the milling vise, but I only skimmed off just enough to clear the casting surface, and get everything level in relation to the casted holes. The cast holes are rough, so the only goal was to get them close, and take as little off as possible. The "feet" are the machined surfaces closest to the camera.
The next step was to machine the "inside" surfaces of the castings. I did each one individually by orienting the foot/feet of each casting to the fixed jaw, and then bumping the part around to get the cast bores as vertical as possible. They are rough casts, but I was able to get it all within an acceptable vertical orientation, and then used a carbide face mill to deck the inside sides. I am planning on JB Welding the 2 pieces together soon, for boring out the main base bar bores, in order to get them perfectly concentric.
The cast holes are decently accurate, but not perfect. By epoxying the parts together and boring both in the same vice set-up, I can guarantee them to be concentric and parallel to each other at least to a tolerance as accurate as my machine is.
I will JB Weld them together while held in orientation by a decent fitting pin, prior to the boring operations. Later I can weaken the epoxy with a torch, and separate the two pieces. Boring will be done following an order of operations where I can implement cotters for clamping in lieu of splitting the castings. The cotter holes would need to be bored first, and then the cotter brass would be lock titan in prior to boring the main base bar holes.
I have not epoxied them together yet. Trying to make sure I am not painting myself into a corner, so order of operations will be critical for all cast parts during this project. The base castings will look like this when epoxied, and then the orientation bar will be slid out. At this point, the 3 feet will be my primary reference plane till the parts are again separated.