I changed Bob's design a bit to accommodate a 35"x24" Birmingham milling machine base. The caster mount channels are 23 inches long and the crossbeams are 46 inches long (be sure to calculate caster swivel clearance).
Note: Depending on your steel prices I would consider using 2"x3"x1/4" tubing instead of using 2"x3" angle stock welded to make the crossbeam and for the caster mounts consider 4"x1/4" channel instead of 2"x2"x1/4" angle welded together to make a channel. By substituting tubing and channel for the angle stock in this project the only welding required would be on the 2"x3"x1/4" angle lips of the crossbeams.
Another design change is bolting the crossbeam to the caster mounts instead of welding, this allows disassembly for easier storage. This change also allows multiple crossbeams of varying widths without needing to build a whole new set of carriages.
The last design change was eliminating the gusset behind the 2"x3" angle lips on the crossbeams. Loaded with a 2400 lb machine there was no noticeable deflection in the lips
These puppies are heavy duty, my 2400 lb Birmingham mill and 2200 lb JET lathe didn't stress them in the least. If you know you aren't going to move anything over 3 thousand pounds I would consider using 3/16" stock instead of 1/4".
I hope you find these photos informative and helpful.
Disclaimer: I in no way endorse the safety of this project. This is a tool made for my own use in performing an inherently risky task. I can not and will not be responsible for you or anyone else that gets hurt in any way. If you try something similar, be sure to use good common sense.
The trusty Northern Tool 4x6 bandsaw hard at work.
Welding up the caster channels. The channels are made from 2"x2"x1/4 angle.
I lucked out and found the heavy duty 4"x2" casters (also heavy to ship) at our local Northern Tool store at a cost of US$5.99 each 
They have ball bearing swivels and roller bearings on the axals.
Finished welding up the first crossbeam.
Hint... check the price of 2"x3"x1/4" tubing vs. angle stock, the tubing would eliminate a lot of setup and welding. Same goes for the 4"x1/4" caster channels. I love to weld and needed practice MIG welding which is new to me. I'm an old timer stick and TIG weldor that is learning to love MIG welding too. So the setup and welding was fun for me.
Marking the location for the crossbeam brackets. The brackets in the photo are 2"x3"x1/4" angle. I think 3"x3"x1/4" would be a better choice here, but I didn't have enough 3"x3" angle on hand. The 2"x3" angle worked fine though.
