I agree, I would advise you do not even try to start the engine without the exhaust system fully supported with the appropriate brackets. Don't even fire it up 'just to check', as the greatest torque of the unsupported exhaust system weight on the cylinder head elbows and studs will be when the engine first has to accelerate from rest to running. You risk snapping the studs holding the elbows into the cylinder head and that means the head off and drilling out the stud holes.