Saw this on CL here in Seattle

Cast Iron Layout/inspection Plate

10-12000 lb layout table? I don't know the difference between a surface plate and a layout table so not sure how 'flat' it is but, jeez, that's a big chunk of cast iron! i'm assuming you could have it ground to be flat enough for frame building.

Anyway, it wouldn't fit in the trunk of my car so, if anyone is interested, I won't be hurt if you steal it from me...