This is some mass-produced model, and it will work. The 11" long one works very well dimensionally for the Grand Daddy front steering box area. At least its what I use after looking around alot. You can probably find it slightly cheaper:
https://www.mooreparts.com/latest-rage-425145-end-load-11-rack-pinion-5-8-36-spline-vw-sand-rail-buggy/?srsltid=AfmBOooTzSbWGNNgkh2GWMjt1jFYAj9D0pD4L-EeOu60jWi1n-AZ31d5Qts
The Heim joints are a little small (3/8" or so bolt), but the actual Steel shaft with the gears/notches is steel, so I went ahead and drilled/re-tapped to 1/2" Heims. Probably overkill, but somebody else on here had an issue, so I copied their fix. It seems fairly solid. I am not sure if you have built your steering area yet, but if you feel comfortable mixing it up, you should try lengthening and widening the box coming off the main frame. You will have more options down the road for Bigger A arms/susspension/bigger tires.
http://essaypapers.reviews/ The build plans are a bit cramped up there in my opinion.