How installed machinery loads a ship's hull

Can installing all of a ship's machinery while it is still on the slipway stress the hull?

It can, especially with off-center weight, but builders design the hull supports and structure to carry those loads.

Machinery can stress a hull when its weight is concentrated far from the center. The effect depends on the hull design, its supports, and the construction practices of the period. Builders normally arrange the slipway supports and struts to carry the machinery weight.