How much does a mostly-stripped A-body weigh?

So I have casters (wttool.com, 600-lb load, swivel, two-axis locking caster, $20 each) and found a hunk of 1/8", 4x2 square-tube in my garage. I also have a hunk of 4", 1/8"-thick flat stock. I'm guessing that's enough to build the bases. I'm probably going to use 2", 1/8" thick angle iron for the top of the fronts, with holes drilled in the horizontal face to mount to the K-member holes and holes drilled in the vertical face to mount to the core support holes. That's for the front end. My question is...What do you do in the back?

I thought about notching/drilling the tube to allow me to bolt to the front spring shackle. Others suggest bolting to the rear bumper support...But I'm not sure about that. It seems iffy to me to hang the weight off of bolt holes that weren't intended to hold more than the weight of a bumper. What say you all?

as long as the car has frame connectors there is no issues... we do most of the reassebly on the rotssery... everything except engine/trans and rear end...