You don't show the configuration of the front ramps. If you put the front wheels on a sloping surface when you jack up the rear it will want to roll down the ramp. I nearly killed myself doing this with my 52 pontiac ambulance.

I recommend taking the time to put all four corners of the car on jack stands....and using quality jack stands rated for plenty of weight.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.