I am designing a fixture to put together a large assembly that is part of a construction vehicle. The assembly is 8'w, 8'h, 3'deep and almost 4000lbs. The only attachment points are two 3/4" bolts 7" apart, patterned at both ends (4 bolts total on the assembly) on the bottom of the assembly. The only part of the assembly that actually contacts the ground are the two 14" x 2 3/8" blocks that the aforementioned bolts fasten to.

The Center of Gravity is pretty well centered between the bolts 4' high (about dead-center of the assembly).

What I need to do:
I'm looking to create a base that will hold the assembly firmly in place while 500lb subassemblies are moved into place and torqued in. Since the assembly is 2 tons and the base of the part is only 14" deep my initial thought was to create a base out of structural steel that was longer along the floor in each direction than the height of the COG from the ground (i.e. 4'+ long in front of the assembly and 4'+ long in the back of the assembly). As floor space is valuable in our facility I do not want to take any larger of a footprint than necessary.

With all my Googling about outriggers, stability, fixture design, moment of inertia, etc., I have not been able to come up with any useful information that would help to define what a footprint for a stable fixture would look like. I.e. what is the relationship to the Center of Gravity, etc?