Your stress is concentrated on the bolt attach points. This is where over time depending how you drive the car you will likely get cracking looseness and failure.
Sure cracking can start at a weld seam, but is less likely because the entire structure is more rigid.
As far as bokts go, case in point, bridges are bolted together and designed to flex so they dont fail. You are bolting together a frame connector to a car chassis with the intent to make it function as 1 solid structure (no flex, or minimum flex).
Will it work? Sure. It will stiffen it up. Will it work as well as a welded in set of connectors ? No over time it will not. It will still allow a bit of flex, and along with that the stress loading and unloading of the bolts.
Over time the clamping force of the bolts will become less as the oem structure deforms behind them, along with cracking from stress loading and unloading.