Need help on welding a broken 150lb vise slide

[NortonDommi]

I take it that you don't have the missing bit? I can see nothing wrong with the plan. Have you thought about replacing the missing bit with Cast Iron?
There are usually old stoves kicking around somewhere with Cast Iron sides that would be of a thickness close to what is need although a piece that had to
be machined to size would be best.
Cast Iron is not that hard to weld with Oxy/Acet. Abrasive blasting and cleaning the weld area, using the correct flux and pre and post heat are necessary.
Most welding shops sell bare Cast Iron rod which is easier than old piston rings, broken scrap etc.
The advantage of welding/Brazing with a Cast Iron rod is that it machines the same and the colour match is perfect. Stippling with a needle scaler or a light sand after grinding/machining to conture makes the repair undetectable.Datasheet_MG240 Bare Cast Iron Rod.pdf