Four decent axle stands under the chassis are probably even better than the wheels on - no movement at all. I would have no hesitation putting the engine in with them holding the chassis. Probably better if the front ones were reasonably close to the front of the chassis, but if you have the springs on, then behind the rear of the front springs should be OK (although under the bumper, if fitted, is perhaps better) particularly if the gearbox is in or is going in with the engine.
John

