That's the plan, tack then short weld, jump around to mitigate warp.
Plan on using the hull as the initial jig then bolt to another aluminum pump for heat soak and keep everything true.
My ski uses 8mm bolts to hold the pump in, so the mounting ears have slop which I've seen cause the pump to slide back and loose seal to duct shoe when only using 2 bolts. Plus I dont want issues with breaking the reduction nozzle and spread the load being only on 8mm bolts.