I think you guys are right about two important aspects here.

1) Steel is springy stuff and transfers a vibrations as good as it does any other energy.

2) build volume: go big, or go home.

To address issue number one I have shifted my focus from an all steel frame to a hybrid steel and "epoxy granite" machine. While I like the idea of having something cast by a foundry, it isn't DIY enough for me. I really enjoy the build process. Maybe more than using the actual machine. Epoxy granite machines seem to have great vibration damping, AND rigidity. or maybe a decent compromise between the two. This method will also enable me to form some pretty complex and large components more easily. I have ordered a couple gallons of epoxy resin and have all sort of play sand around. I might upgrade some parts of my cnc router to experiment with this method. If I am being honest here though, I will probably just jump in feet first and start building a new machine. That is once I have addressed the second issue here.

Build volume. Shoot its easy to go straight gangbusters here and start buying 4000 mm linear rails... but how much is too much? I suppose my limiting factors would be machine foot print/height, and power ( stepper power and power supply to machine). First the physical volume isn't a huge deal as I am a one man household with a decent sized garage. My garage is somewhere in the 25 x 25 neighborhood, although I would prefer this thing to not take up a huge portion of said garage as I have a few other toys that like to call this same structure home. The second limiting factor is power. I am planning to use NEMA 34 closed loop steppers (orrrrrr maybe open loop with linear scales as encoders). as I increase the build volume I will also increase how much mass these bad boys will have to throw around which at some point will start to limit machine speed. I would prefer to over engineer in the stepper department and stay as far away from them losing steps as possible, while keeping what I would consider a reasonable machine speed. I learned on my first CNC router build there is nothing worse than a painfully slow machine. To be quite honest I have no idea how to calculate how much mass is too much, nor do I plan to. So in this aspect I will probably air to the side of caution and keep the build volume smaller than the top fueled maniac inside me screams for. I also have a CNC router that has about a 3' x 4' work volume and handles aluminum like a champ for the most part. Assuming I use due care and caution while calculating tool paths.
My other issue here is I started this out looking at buying a harbor freight bench top mill and converting it. Yet I sit here planing to build a friggen monster machine as I just can't help myself but live in excess one way or another. What do you guys thing? 20x16? I mean a PM-30 is around 30x18. I wouldn'y be mad about something like a 30x20 even. I could use multiple setups that way and machine cylinder heads if I wanted to. I am leaning toward the 30x20 here...

A couple other thoughts. I will be enclosing this machine as I would like to contain the chips and coolant. That will undoubtedly increase the overall size of this thing. Power supply to the machine isn't a huge deal as long as I stick with single phase. My garage is in desperate need of some more angry pixies anyway. My brother and I are getting ready to run a 200 amp supply to the garage while upgrading my house panel as well. My heater keeps tripping the main breaker in my house and that is super annoying.

Also, thanks for the feedback. I am greatly appreciative for it!